math.php

<?php
class Math
{
    var $operations = array(
        '+' => 'plus',
        '-' => 'minus',
        '*' => 'multiply',
        '/' => 'devide',
        // add more functions here
    );

    /**
     * Parsing and calculating Reverse Polish notation (RPN)
     *
     * Example:
     * $math = new Math;
     * echo $math->parse('1 2 + 4 + 2 * 7 /');
     */
    public function parse ($str) {
        $stack = array();

        $token = strtok($str, ' ');

        while ($token !== false)
        {
            if (in_array($token, array_keys($this->operations)))
            {
                if (count($stack) < 1) {
                    throw new Exception("Luck of data in token: '$token'");
                } else if (count($stack) > 2) {
                    $b = array_pop($stack);    
                } else {
                    $b = null;
                }
                
                $a = array_pop($stack);
                $res = $this->calculate($a, $b, $token);
                array_push($stack, $res);
            } elseif (is_numeric($token))
            {
                array_push($stack, $token);
            } else
            {
                throw new Exception("Wrong symbol: $token");
            }

            $token = strtok(' ');
        }
        if (count($stack) > 1)
            throw new Exception("Operators doesn't much operands");
        return array_pop($stack);
    }

    protected function calculate ($a, $b, $operation) {
        if (!$operation || !isset($this->operations[$operation])) {
            throw new Exception('Operation should be specified');
        }
        $method = $this->operations[$operation];
        try {
            if ($b == null) {
                $result = $this->$method($a);    
            } else {
                $result = $this->$method($a, $b);    
            }            
        } catch (Exception $e) {
            throw new Exception($e->getMessage());
        }
        return $result;
    }

    protected function plus ($a, $b) {
        return $a + $b;
    }

    protected function minus ($a, $b) {
        return $a - $b;
    }

    protected function devide ($a, $b) {
        if ($b == 0) {
            throw new Exception ('Devision by zero');
        }
        return $a / $b;
    }

    protected function multiply ($a, $b) {
        return $a * $b;
    }
}