如何编写可读的if语句进行输入验证？

if ( $validation->run() === true ) {

    if ( Diesel::usernameExists( $username ) === false ) {

        $error = \'This username is already taken! Try again...\';

    }

    if ( Diesel::emailExists( $email ) === false ) {

        $error = \'This e-mail is already taken! Try again...\';

    }


    if ( !isSet( $error ) ) {

        Diesel::signUp( $username, sha1( $password ), $email );


        Session::set_flash( \'notification\', \'You have been successfully signed-up! Thanks...\' );
        Session::set_flash( \'location\', \'sign-in\' );

        $this->response->redirect( \'notification\' );

    }

} else {

    Session::set_flash( \'error\', \'There was something wrong with validation! Try again...\' );

    $this->response->redirect( \'sign-up\' );

}

$validation->run()

usernameExists()

emailExists()