Wordpress - Check if wp-login is current page

Use the global $pagenow, which is a common global set by WordPress at runtime:

if ( $GLOBALS['pagenow'] === 'wp-login.php' ) {
    // We're on the login page!
}

You can also check the type of login page, for example registration:

if ( $GLOBALS['pagenow'] === 'wp-login.php' && ! empty( $_REQUEST['action'] ) && $_REQUEST['action'] === 'register' ) {
    // We're registering
}

Following code is considered legacy and should not be used (wp-register.php was deprecated & subsequently removed quite a while back):

if ( in_array( $GLOBALS['pagenow'], array( 'wp-login.php', 'wp-register.php' ) ) )
    run_my_funky_plugin();

My preferred way:

if( is_wplogin() ){
   ...
}

code:

function is_wplogin(){
    $ABSPATH_MY = str_replace(array('\\','/'), DIRECTORY_SEPARATOR, ABSPATH);
    return ((in_array($ABSPATH_MY.'wp-login.php', get_included_files()) || in_array($ABSPATH_MY.'wp-register.php', get_included_files()) ) || (isset($_GLOBALS['pagenow']) && $GLOBALS['pagenow'] === 'wp-login.php') || $_SERVER['PHP_SELF']== '/wp-login.php');
}

Why it's safest?

1. Sometimes, if you try to check login page using REQUEST_URI(or SCRIPT_PATH), you will get INCORRECT VALUES, because many plugins change LOGIN & ADMIN urls.
   2) $pagenow will give you incorrect value too in that case!

Notes:

• In some cases, it might not work if you output login-form (i.e. with shortcode or etc) manually on other template files/pages.

More modern way to do that, it should work even when the wp-login URL is changed by plugins and when WP is in a subfolder, etc:

if(stripos($_SERVER["SCRIPT_NAME"], strrchr(wp_login_url(), '/')) !== false){
    /* ... */
}