Math Solver - Shunting Yard

<?php

function tokenize($expr) {
    preg_match_all('/(?:-|)\d+|\+|-|\*|\/|\(|\)/', $expr, $match);
    $match  = $match[0];
    $tokens = array();
    $stack  = array();
    $precedence = array('+' => 1, '-' => 1, '*' => 2, '/' => 2);

    while($match) {
        $token = array_shift($match);

        if(is_numeric($token)) {
            array_push($tokens, $token);
        }
        else {
            if($token === ')') {
                while($stack && end($stack) !== '(') {
                    array_push($tokens, array_pop($stack));
                }

                array_pop($stack);
                continue;
            }

            while($token !== '(' && ($stack && end($stack) !== '(') && $precedence[$token] < $precedence[end($stack)]) {
                array_push($tokens, array_pop($stack));
            }

            array_push($stack, $token);
        }
    }

    return array_merge($tokens, array_reverse($stack));
}

function resolve($math) {
    $funcs = array(
        '+' => function ($left, $right) { return $left + $right; },
        '-' => function ($left, $right) { return $left - $right; },
        '/' => function ($left, $right) { return $left / $right; },
        '*' => function ($left, $right) { return $left * $right; }
    );

    $stack = array();

    while(count($math) > 1) {
        $token = array_shift($math);

        if(!is_numeric($token)) {
            $token = $funcs[$token](array_shift($math), array_shift($math));
        }

        array_push($stack, $token);
    }

    return $stack[0];
}

header('content-type: text/plain');
$math = tokenize('2*3+4*(5 - 6)');
echo implode(' ', $math) . PHP_EOL;
echo resolve($math);

?>