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Depending on your needs, I would suggest looking into the <a href="http://en.wikipedia.org/wiki/Shunting-yard_algorithm">Shunting Yard Algorithm</a>. It's pretty easy to implement, and works quite well. Here's an example I whipped up a while ago: <a href="https://gist.github.com/1232629">GIST</a>. Here's the code copy/pasted into one block: Expression Definitions: <pre><code>class Parenthesis extends TerminalExpression { protected $precidence = 7; public function operate(Stack $stack) { } public function getPrecidence() { return $this->precidence; } public function isNoOp() { return true; } public function isParenthesis() { return true; } public function isOpen() { return $this->value == '('; } } class Number extends TerminalExpression { public function operate(Stack $stack) { return $this->value; } } abstract class Operator extends TerminalExpression { protected $precidence = 0; protected $leftAssoc = true; public function getPrecidence() { return $this->precidence; } public function isLeftAssoc() { return $this->leftAssoc; } public function isOperator() { return true; } } class Addition extends Operator { protected $precidence = 4; public function operate(Stack $stack) { return $stack->pop()->operate($stack) + $stack->pop()->operate($stack); } } class Subtraction extends Operator { protected $precidence = 4; public function operate(Stack $stack) { $left = $stack->pop()->operate($stack); $right = $stack->pop()->operate($stack); return $right - $left; } } class Multiplication extends Operator { protected $precidence = 5; public function operate(Stack $stack) { return $stack->pop()->operate($stack) * $stack->pop()->operate($stack); } } class Division extends Operator { protected $precidence = 5; public function operate(Stack $stack) { $left = $stack->pop()->operate($stack); $right = $stack->pop()->operate($stack); return $right / $left; } } class Power extends Operator { protected $precidence=6; public function operate(Stack $stack) { $left = $stack->pop()->operate($stack); $right = $stack->pop()->operate($stack); return pow($right, $left); } } abstract class TerminalExpression { protected $value = ''; public function __construct($value) { $this->value = $value; } public static function factory($value) { if (is_object($value) && $value instanceof TerminalExpression) { return $value; } elseif (is_numeric($value)) { return new Number($value); } elseif ($value == '+') { return new Addition($value); } elseif ($value == '-') { return new Subtraction($value); } elseif ($value == '*') { return new Multiplication($value); } elseif ($value == '/') { return new Division($value); } elseif ($value == '^') { return new Power($value); } elseif (in_array($value, array('(', ')'))) { return new Parenthesis($value); } throw new Exception('Undefined Value ' . $value); } abstract public function operate(Stack $stack); public function isOperator() { return false; } public function isParenthesis() { return false; } public function isNoOp() { return false; } public function render() { return $this->value; } } </code></pre> The stack (really simple implementation): <pre><code>class Stack { protected $data = array(); public function push($element) { $this->data[] = $element; } public function poke() { return end($this->data); } public function pop() { return array_pop($this->data); } } </code></pre> And finally, the executor class: <pre><code>class Math { protected $variables = array(); public function evaluate($string) { $stack = $this->parse($string); return $this->run($stack); } public function parse($string) { $tokens = $this->tokenize($string); $output = new Stack(); $operators = new Stack(); foreach ($tokens as $token) { $token = $this->extractVariables($token); $expression = TerminalExpression::factory($token); if ($expression->isOperator()) { $this->parseOperator($expression, $output, $operators); } elseif ($expression->isParenthesis()) { $this->parseParenthesis($expression, $output, $operators); } else { $output->push($expression); } } while (($op = $operators->pop())) { if ($op->isParenthesis()) { throw new RuntimeException('Mismatched Parenthesis'); } $output->push($op); } return $output; } public function registerVariable($name, $value) { $this->variables[$name] = $value; } public function run(Stack $stack) { while (($operator = $stack->pop()) && $operator->isOperator()) { $value = $operator->operate($stack); if (!is_null($value)) { $stack->push(TerminalExpression::factory($value)); } } return $operator ? $operator->render() : $this->render($stack); } protected function extractVariables($token) { if ($token[0] == '$') { $key = substr($token, 1); return isset($this->variables[$key]) ? $this->variables[$key] : 0; } return $token; } protected function render(Stack $stack) { $output = ''; while (($el = $stack->pop())) { $output .= $el->render(); } if ($output) { return $output; } throw new RuntimeException('Could not render output'); } protected function parseParenthesis(TerminalExpression $expression, Stack $output, Stack $operators) { if ($expression->isOpen()) { $operators->push($expression); } else { $clean = false; while (($end = $operators->pop())) { if ($end->isParenthesis()) { $clean = true; break; } else { $output->push($end); } } if (!$clean) { throw new RuntimeException('Mismatched Parenthesis'); } } } protected function parseOperator(TerminalExpression $expression, Stack $output, Stack $operators) { $end = $operators->poke(); if (!$end) { $operators->push($expression); } elseif ($end->isOperator()) { do { if ($expression->isLeftAssoc() && $expression->getPrecidence() <= $end->getPrecidence()) { $output->push($operators->pop()); } elseif (!$expression->isLeftAssoc() && $expression->getPrecidence() < $end->getPrecidence()) { $output->push($operators->pop()); } else { break; } } while (($end = $operators->poke()) && $end->isOperator()); $operators->push($expression); } else { $operators->push($expression); } } protected function tokenize($string) { $parts = preg_split('((\d+|\+|-|$|$|\*|/)|\s+)', $string, null, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE); $parts = array_map('trim', $parts); return $parts; } } </code></pre> It works by first tokenizing the input (based on word boundary, and tokens). Then, it runs the Shunting Yard algorithm on it to convert the input into a RPN (Reverse Polish Notation) stack. Then, it's just a matter of executing the stack. Here's a quick example: <pre><code>$math = new Math(); $answer = $math->evaluate('(2 + 3) * 4'); var_dump($answer); // int(20) $answer = $math->evaluate('1 + 2 * ((3 + 4) * 5 + 6)'); var_dump($answer); // int(83) $answer = $math->evaluate('(1 + 2) * (3 + 4) * (5 + 6)'); var_dump($answer); // int(231) $math->registerVariable('a', 4); $answer = $math->evaluate('($a + 3) * 4'); var_dump($answer); // int(28) $math->registerVariable('a', 5); $answer = $math->evaluate('($a + $a) * 4'); var_dump($answer); // int(40) </code></pre> Now, this example is significantly more complex than you may need. The reason is that it also handles grouping and operator precedence. But it's a decent example of a running algorithm that doesn't use EVAL and supports variables...
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