leetcode28. 实现strstr()

function strStrN(string $haystack, string $needle) :int {
    $lengthNeedle = strlen($needle);

    if ($lengthNeedle == 0) {
        return 0;
    }

    $lengthHaystack = strlen($haystack);

    $i = $j = 0;

    while ($i <= $lengthHaystack - $lengthNeedle) {
        for ($j = $lengthNeedle - 1; $j >= 0; $j--) {
            if ($haystack[$i + $j] != $needle[$j]) {
                break;
            }
        }

        if ($j == -1) {
            return $i;
        }

        $i++;
    }

    return -1;
}