Fix incorrect client IP for WordPress behind reverse proxy

If you are running WordPress behind a reverse proxy, you will need to add the below code to the top of wp-config.php:

//   Alter `REMOTE_ADDR` header to `HTTP_X_FORWARDED_FOR` if it exists. 
if($_SERVER['HTTP_X_FORWARDED_FOR']) {
    $xaddr = explode(',',$_SERVER['HTTP_X_FORWARDED_FOR']);
    $_SERVER['REMOTE_ADDR'] = $xaddr[0];
}