check login URL for specific query string

login-string-check.php

<?php
/*
* Check the URL of the WordPress login
* page for a specific query string
*
* assumes login string is
* http://mysite.com/wp-login.php?question=answer
*/
function rkv_login_stringcheck() {

	// set the location a failed attempt goes to
	$redirect = 'http://www.google.com/';

	// missing query string all together
	if (!isset ($_GET['question']) )
		wp_redirect( esc_url_raw ($redirect), 302 );

	// incorrect value for query string
	if ($_GET['question'] !== 'answer' )
		wp_redirect( esc_url_raw ($redirect), 302 );

}

add_action( 'login_init', 'rkv_login_stringcheck' );