I thought I'd make a little script that takes advantage of this easteregg and the inherent format of GIF image files to determine whether PHP is running on a remote server.

<?php

	// Configuration.
	$url = 'http://www.example.com/'; // Replace with whichever URL.

	// Open the connection.
	$handle = @fopen($url.'?=PHPE9568F36-D428-11d2-A769-00AA001ACF42', 'r');
	if($handle)
		echo("Handle active...\n"); else
		die("Error creating handle!\n");

	// Import data into buffer.
	$buffer = fgets($handle, 4);
	if($buffer == 'GIF') // <- part of the binary header for GIF files.
		echo("This server is using PHP!\n"); else
		echo("This server is NOT using PHP!\n");

	// Close up, clean up.
	$ret = fclose($handle);
	$handle = NULL;
	if($ret)
		echo('Success!'); else
		die('Failed to close handle!');

?>

Don't forget to replace $url with the server address before using this script.