The login_head filter can be used to filter the logo image on the WordPress login page. By default this logo is of WordPress.
Note: this is not the only possible use of this filter. It can be used to add anything to the <head> section on the login page.
<?php add_filter( 'login_head', 'custom_function_name' ); ?>
Where "custom_function_name" is the function to be called when the content is being retrieved.
In the below example the default logo is changed to custom logo, using CSS.
<?php
function my_custom_login_logo() {
echo '<style type="text/css">
h1 a {background-image:url(http://example.com/your-logo.png) !important; margin:0 auto;}
</style>';
}
add_filter( 'login_head', 'my_custom_login_logo' );
?>
To validate the login details before user logins, you can use the below function :
(p.s. At first, you may have inserted some extra fields using 'login_form' action hook.)
function ref_access() {
global $error;
if(empty($_POST['custom_field_name']))
{
$error = 'Restricted area, please login to continue.';
}
}
add_action('login_head','ref_access');