How to evaluate formula passed as string in PHP?

Question

Just trying to figure out the proper and safer way to execute mathematical operation passed as string. In my scenario it is values fetched from image EXIF data.

After little research I found two way of doing it.

first, using eval:

function calculator1($str){     eval("\$str = $str;");     return $str; } 

second, using create_function:

function calculator2($str){     $fn = create_function("", "return ({$str});" );     return $fn(); }; 

Both examples require string cleanup to avoid malicious code execution. Is there any other or shorter way of doing so?

Answer 1

This might help.

http://www.phpclasses.org/browse/package/2695.html

Annoying login required to download. I copied an pasted it here for you.

• This class can be used to safely evaluate mathematical expressions.
• The class can take an expression in a text string and evaluate it by replacing values of variables and calculating the results of mathematical functions and operations.
• It supports implicit multiplication, multivariable functions and nested functions.
• It can be used to evaluate expressions from untrusted sources. It provides robust error checking and only evaluates a limited set of functions.
• It could be used to generate graphs from expressions of formulae.

    /*     ================================================================================      EvalMath - PHP Class to safely evaluate math expressions     Copyright (C) 2005 Miles Kaufmann <http://www.twmagic.com/>      ================================================================================      NAME         EvalMath - safely evaluate math expressions      SYNOPSIS         <?           include('evalmath.class.php');           $m = new EvalMath;           // basic evaluation:           $result = $m->evaluate('2+2');           // supports: order of operation; parentheses; negation; built-in functions           $result = $m->evaluate('-8(5/2)^2*(1-sqrt(4))-8');           // create your own variables           $m->evaluate('a = e^(ln(pi))');           // or functions           $m->evaluate('f(x,y) = x^2 + y^2 - 2x*y + 1');           // and then use them           $result = $m->evaluate('3*f(42,a)');         ?>      DESCRIPTION         Use the EvalMath class when you want to evaluate mathematical expressions          from untrusted sources.  You can define your own variables and functions,         which are stored in the object.  Try it, it's fun!      METHODS         $m->evalute($expr)             Evaluates the expression and returns the result.  If an error occurs,             prints a warning and returns false.  If $expr is a function assignment,             returns true on success.          $m->e($expr)             A synonym for $m->evaluate().          $m->vars()             Returns an associative array of all user-defined variables and values.          $m->funcs()             Returns an array of all user-defined functions.      PARAMETERS         $m->suppress_errors             Set to true to turn off warnings when evaluating expressions          $m->last_error             If the last evaluation failed, contains a string describing the error.             (Useful when suppress_errors is on).      AUTHOR INFORMATION         Copyright 2005, Miles Kaufmann.      LICENSE         Redistribution and use in source and binary forms, with or without         modification, are permitted provided that the following conditions are         met:          1   Redistributions of source code must retain the above copyright             notice, this list of conditions and the following disclaimer.         2.  Redistributions in binary form must reproduce the above copyright             notice, this list of conditions and the following disclaimer in the             documentation and/or other materials provided with the distribution.         3.  The name of the author may not be used to endorse or promote             products derived from this software without specific prior written             permission.          THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR         IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED         WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE         DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,         INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES         (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR         SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)         HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,         STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN         ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE         POSSIBILITY OF SUCH DAMAGE.      */      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];         }     } 

EDIT: Jitters wanted the version that supports reverse polish notation. Reminds me of my college days when I had an HP calculator :)

    <?php     /* This Class can be useful for writting RPN macros or FORTH like parsers             @Author: Arturo Gonzalez-Mata Santana (Spain)                  [email protected]         @copyright 2007: www.phpsqlasp.com          It is part of a project to recover "macros" from some old aplications       This code is free software; you can redistribute it and/or     modify it under the terms of the GNU General Public License     as published by the Free Software Foundation; either version 3     of the License, or (at your option) any later version.      This program is distributed in the hope that it will be useful,     but WITHOUT ANY WARRANTY; without even the implied warranty of     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the     GNU General Public License for more details.      You should have received a copy of the GNU General Public License     along with this program; if not, write to the Free Software     Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA     */     class RPNstack     {         var $data=array();         var $compare=0;         function pop() {return array_shift ($this->data);}         function push($x) {array_unshift($this->data, $x);}         function count() {return count($this->data);}         function first() {return $this->data[0];}         function top() {return end($this->data);}  //last element of          function swap() { // interchange tow elements             $t = $this->data[1];             $this->data[1] = $this->data[0];             $this->data[0] = $t;             }         function dup() {  // put a copy of X element in the stack             array_unshift($this->data, $this->data[0]);             }          function dump(){ // dump array data for debuging             print_r($this->data);             }          function parse($tok)  // execute actions with the stack for each token         {             $r = null;             $tok = strtoupper(trim($tok));             //$this->dump(); // this line is for debugging purpose only             switch ($tok) :                 // FIRST "IF THEN" AND OTHER FLOW CONTROLS                 case ('THEN'): break;                 case('IF'):                     if ($this->pop() == 0) do {  // if condition is false do nothing until "THEN"                         $tok = strtoupper(strtok (" "));                         } while ($tok <> "THEN");  // IF THERE IS NO "THEN" THIS SHALL BE AN ENLESS LOOP                     break;                  //   basic math operators   //OPERADORES MATEMATICOS BASICOS                 case('+'):                     $r = $this->pop() + $this->pop();                     // $r = array_shift($this->data) + array_shift($this->data);  // is more efficient but less understable                     break;                         case('-'):                     $r = $this->pop(); $r = $this->pop()-$r;                     break;                 case('*'):                     $r = $this->pop() *  $this->pop();                     break;                 case('/'):                     $r = $this->pop(); $r = $this->pop() / $r;                     break;                 // stack operators  //OPERADORES DE PILA                   case ('DUP'):                     $r=$this->dup();                     break;                 case ('SWAP'):                     $this->swap();                     break;                  // COMPARISON OPERATORS                 case ('='):                     if ($this->data[0] == $this->data[1]) $r = $this->push(1);                     else $r = $this->push(0);                     break;                 case ('<>'):                     if ($this->data[0] <> $this->data[1]) $r = $this->push(1);                     else $r = $this->push(0);                     break;                 case ('<'):                     if ($this->data[0] < $this->data[1]) $r = $this->push(1);                       else $r = $this->push(0);                                                 break;                 case ('>'):                     if ($this->data[0] > $this->data[1])  $r = $this->push(1);                       else $r = $this->push(0);                                                 break;                 case ('>='):                     if ($this->data[0] >= $this->data[1])  $r = $this->push(1);                       else $r = $this->push(0);                                                 break;                 case ('<='):                     if ($this->data[0] <= $this->data[1])  $r = $this->push(1);                       else $r = $this->push(0);                                                 break;                  // WARNING FOR NON IMPLEMENTED FUNCTIONS                 default:                         return sprintf('I don\'t know how to "%s" ', $tok);             endswitch;             if (!is_null($r)) $this->push($r);              return $r;         } // parse           function parse_line($cadena)         {             $tok = strtok ($cadena," ");             while ($tok!= '') {                 if (is_numeric ($tok)) {                     $this->push($tok);                 } else {                     $r = $this->parse($tok);                 }                 $tok = strtok (" ");             }             return $r;         }      } // class RPN        ?>