public function signIn(){
    $allowed = ['user', 'password'];
    $success = false;

    if($_POST){
        foreach($allowed as $a){
            if(isset($_POST[$a])){
                $success = !empty($_POST[$a]) ? true : false;
                break;
            }else{
                $success = false;
                break;
            }
        }
        if($success){
            $user = stripslashes(strip_tags(rtrim($_POST[$allowed['user']])));
            $pass = hash('sha512', stripslashes(strip_tags(r_trim($_POST[$allowed['password']]))));
            echo 'ok';
            }else{
                $errors[] = 'User or password incorrect, please retry!';
            }
        }else{
            $errors[] = 'Post data are invalid, refresh page and try i!';
        }
    }else{
        $errors[] = "Post data are empty, refresh page and try i!";
    }
    echo $errors[0];
}