Einfache Rechenoperationen

<?php

$mathString = "-5.5*3*-20/5+81-3-5+4*-3.141529*-10";

function replace_math($matches) {
    if ($matches[4] == '-') {
        $vals = explode('!', preg_replace('/(--)*-(-?)/', '!$2', $matches[0]));
    }else {
        $vals = explode($matches[4], $matches[0]);
    }
    switch($matches[4]) {
        case '*':
            return array_product($vals);
        case '/':
            $result = array_shift($vals);
            foreach($vals as $val) {$result/=$val;}
            return $result;
        case '+':
            return array_sum($vals);
        case '-':
            return array_shift($vals)-array_sum($vals);
    }
}
$result = preg_replace_callback("#(?<![0-9])([-+]?([0-9]*\.[0-9]+|[0-9]+))(([/])([-+]?([0-9]*\.[0-9]+|[0-9]+)))+#", 'replace_math' ,$mathString);
$result = preg_replace_callback("#(?<![0-9])([-+]?([0-9]*\.[0-9]+|[0-9]+))(([*])([-+]?([0-9]*\.[0-9]+|[0-9]+)))+#", 'replace_math' ,$result);
$result = preg_replace_callback("#(?<![0-9])([-+]?([0-9]*\.[0-9+]|[0-9]+))(([-])([-+]?([0-9]*\.[0-9]+|[0-9]+)))+#", 'replace_math' ,$result);
$result = preg_replace_callback("#(?<![0-9])([-+]?([0-9]*\.[0-9]+|[0-9]+))(([+])([-+]?([0-9]*\.[0-9]+|[0-9]+)))+#", 'replace_math' ,$result);

eval('$phpResult = '.$mathString.';');
echo $phpResult. " == ".$result;