Math Solver - Shunting Yard v2

<?php

function tokenize($expr) {
    preg_match_all('/(?:-|)\d+|\+|-|\*|\^|\/|\(|\)/', $expr, $match);
    $match  = $match[0];
    $tokens = array();
    $stack  = array();
    $precedence = array('+' => 2, '-' => 2, '*' => 3, '/' => 3, '^' => 4);

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

        if(is_numeric($token)) {
            array_push($tokens, $token);
            continue;
        }

        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; },
        '^' => 'pow'
    );

    $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('3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3');
echo implode(' ', $math) . PHP_EOL;
echo resolve($math);

?>