000-fix-core-42656.php

<?php

/*
Plugin Name: Fix core issue 42656
Description: Fix PHP Warning undefined index pagenow on login popup. Place this into wp-content/mu-plugins/
Author: McGuffin
Author URI: http://github.org/mcguffin
Version: 0.0.1
*/

// borrowed from wp-includes/vars.php
global $pagenow;

// On which page are we ?
if ( is_admin() ) {
	// wp-admin pages are checked more carefully
	if ( is_network_admin() )
		preg_match('#/wp-admin/network/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches);
	elseif ( is_user_admin() )
		preg_match('#/wp-admin/user/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches);
	else
		preg_match('#/wp-admin/?(.*?)$#i', $_SERVER['PHP_SELF'], $self_matches);
	$pagenow = $self_matches[1];
	$pagenow = trim($pagenow, '/');
	$pagenow = preg_replace('#\?.*?$#', '', $pagenow);
	if ( '' === $pagenow || 'index' === $pagenow || 'index.php' === $pagenow ) {
		$pagenow = 'index.php';
	} else {
		preg_match('#(.*?)(/|$)#', $pagenow, $self_matches);
		$pagenow = strtolower($self_matches[1]);
		if ( '.php' !== substr($pagenow, -4, 4) )
			$pagenow .= '.php'; // for Options +Multiviews: /wp-admin/themes/index.php (themes.php is queried)
	}
} else {
	if ( preg_match('#([^/]+\.php)([?/].*?)?$#i', $_SERVER['PHP_SELF'], $self_matches) )
		$pagenow = strtolower($self_matches[1]);
	else
		$pagenow = 'index.php';
}
unset($self_matches);

Last words: This is no longer necessary since https://core.trac.wordpress.org/changeset/44317/trunk/src/wp-includes/l10n.php