WordPress - Setup SSL. Prefer SSL usage for admin area.

wp-config.php

<?php
/**
 * File: wp-config.php
 * 
 * 1. Prefer SSL usage for admin area
 * 2. Check for https existence
 * 
 */

/** wp-config.php / Prefer SSL usage for admin area */
define( 'FORCE_SSL_ADMIN', true );

/** wp-config.php / Check for https existence */
// in some setups HTTP_X_FORWARDED_PROTO might contain 
// a comma-separated list e.g. http,https
// so check for https existence
if (strpos($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') !== false)
$_SERVER['HTTPS']='on';