Balanced unbalanced string

gistfile1.php

<?php

// (if (zero? x) max (/ 1 x))
function balance($str)
{
    function loop($str, $acc)
    {
        if (empty($str)) {
            return $acc == 0;
        }

        $h = head($str);
        if ($h == "(") {
            $n = $acc + 1;
        } elseif($h == ")") {
            $n = $acc - 1;
        } else {
            $n = $acc;
        }

        if ($n >= 0) {
            return loop(tail($str), $n);
        }
        return false;
    }
    return loop($str, 0);
}

function head($str)
{
    return $str[0];
}

function tail($str)
{
    return substr($str, 1);
}

$balance = balance("(if (zero? x) max (/ 1 x))");

if ($balance) {
    echo "balanced\n";
} else {
    echo "unbalanced\n";
}