隨機產生四碼長度的英文

    //產生隨機碼，預設四碼，約七百次會出現碰撞
    function get_random_key($length = 4)
    {
        $permitted_chars = '1234567890abcdefghijklmnopqrstuvwxyxABCDEFGHIJKLMNOPQRSTUVWXYX';
        return substr(str_shuffle($permitted_chars), 0, $length);
    }