<?php
function tokenize($str) {
    $operators = array('(', ')', '+', '-', '*', '/');
    $tokens = array();
    $buffer = '';
    $push = function() use (&$tokens, &$buffer) {
        if(strlen($buffer) > 0) {
            $tokens[] = intval($buffer);
            $buffer = '';
        }
    };

    for($i = 0, $l = strlen($str); $i < $l; $i++) {
        if(is_numeric($str[$i])) {
            $buffer .= $str[$i];
        } else if(in_array($str[$i], $operators)) {
            $push();
            $tokens[] = $str[$i];
        }
    }

    $push();

    return $tokens;
}

function expr($tokens) {
    list($v, $tokens) = term($tokens);

    while(count($tokens) > 0 && in_array($tokens[0], array('+', '-'))) {
        $t = array_shift($tokens);
        list($w, $tokens) = term($tokens);

        if($t == '+') {
            $v += $w;
        } else if($t == '-') {
            $v -= $w;
        }
    }

    return array($v, $tokens);
}

function term($tokens) {
    list($v, $tokens) = factor($tokens);

    while(count($tokens) > 0 && in_array($tokens[0], array('*', '/'))) {
        $t = array_shift($tokens);
        list($w, $tokens) = term($tokens);

        if($t == '*') {
            $v *= $w;
        } else if($t == '/') {
            $v /= $w;
        }
    }

    return array($v, $tokens);
}

function factor($tokens) {
    assert(count($tokens) > 0);
    $t = array_shift($tokens);

    if($t == '(') {
        list($v, $tokens) = expr($tokens);
        $t = array_shift($tokens);
        assert($t == ')');
        return array($v, $tokens);
    } else if($t == '-') {
        $number = array_shift($tokens);
        assert(is_numeric($number));
        return array(-$number, $tokens);
    } else {
        assert(is_numeric($t));
        return array($t, $tokens);
    }
}

function calc($str) {
    list($value, $tokens) = expr(tokenize($str));
    assert(count($tokens) == 0);
    return $value;
}

echo "Ergebnis: " . calc($argv[1]);