There simply isn't any source. Headers can be removed or they can be modified to say a site is using X when it really uses Y. Any other detections have just as

There simply isn't any source. Headers can be removed or they can be modified to say a site is using X when it really uses Y. Any other detections have just as many problems, having files ending in php doesn't mean they are php, even a site that has something like phpversion.php could be just a saved HTML copy. The biggest PHP users on the linked list don't even expose that they use PHP, but it is publicly known (though Facebook uses [or used] their own fork of PHP called HHVM).