校验有效括号

        $str = "([)]";
    var_dump(isValid($str));

    function isValid($s) {
        $map = array(
            "(" => ")",
            "{" => "}",
            "[" =>"]"
        );

        $len = strlen($s);
        $stack = array();
        for ($i = 0; $i < $len; $i ++) {
            if (isset($map[$s[$i]])) {
                array_push($stack, $map[$s[$i]]);
            }else {
                $pop = array_pop($stack);
                if ($pop != $s[$i]) {
                    return false;
                }
            }
        }
        if (empty($stack)) {
            return true;
        }
        return false;

    }