Простой калькулятор с переменными

FieldCalculator.php

<?php
/*
 * This file is part of the <censored> informational system package.
 *
 * (c) Serg N. Kalachev <[email protected]>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */
namespace AppBundle\Util;

/**
* Class provided by http://stackoverflow.com/users/555222/clarkk
* in this answer http://stackoverflow.com/a/27077376
*/
class FieldCalculator {

    public static function callback1($m) {
        return self::calculate($m[1]);
    }

    public static function callback2($n,$m) {
        $o=$m[0];
        $m[0]=' ';
        return $o=='+' ? $n+$m : ($o=='-' ? $n-$m : ($o=='*' ? $n*$m : $n/$m));
    }

    public static function calculate($s){ 
        while ($s != ($t = preg_replace_callback('/\(([^()]*)\)/','self::callback1',$s))) $s=$t;
        preg_match_all('![-+/*].*?[\d.]+!', "+$s", $m);
        return array_reduce($m[0], 'self::callback2');
    }

    public static function resolve_vars_in_string($formula, array $values)
    {
        preg_match_all('/\$([a-zA-Z0-9]+)/', $formula, $out, PREG_PATTERN_ORDER);
        foreach(array_unique($out[1]) as $variable){
            if (!isset($values[$variable])) {
                throw new \LogicException("Unexpected variable '$variable' in formula '$formula'");
            }
            $formula=str_replace('$'.$variable, $values[$variable], $formula);
        }

        return $formula;

    }
}

test.php

<?php 

use AppBundle\Util\FieldCalculator;

$formula = '$a * $b * 21'; 
$params = ['a' => 4, 'b' => 0.5];
$formula = FieldCalculator::resolve_vars_in_string($formula, $params);
echo FieldCalculator::calculate($formula); // 42