gistfile1.txt

if(
strpos( $_SERVER['HTTP_REFERER'], 'wp-admin' ) === false &&
strpos( $_SERVER['REQUEST_URI'], 'admin-ajax.php' ) !== false
) {
header( 'Cache-Control: max-age=30000, must-revalidate' );
header( 'Expires: ' . gmdate( 'D, d M Y H:i:s', strtotime( '+5000 minutes' ) ) . ' GMT' );
header( 'Last-Modified: ' . gmdate( 'D, d M Y H:i:s', strtotime( '-5000 minutes' ) ) . ' GMT' );
header( $_SERVER["SERVER_PROTOCOL"]." 404 Not Found" );
die;
}