But how many php websites are there on the web?
Oh, I dunno -- 80% or more of websites out there right now? What with Wordpress, Magento, Codeignitor, vBulletin, phpBB, Xenforo, myBB, SMF, Drupal, Serendipity, Joomla... ALL being PHP based? I mean seriously, with the sheer volume of turdpress sites being crapped out by people who have no business making websites ALONE?
Honestly, websites serving flat .html files (or the twenty-five year out of date windows 3 era .htm) are pretty much in the minority now. Between PHP and ASP, you've probably got more than 95% of deployed websites covered... you toss in Ruby, Perl and Python, that's probably the majority of that remaining 5%!
EVEN if you don't see a .asp or .php extension, since a lot of people are using rewrite-rules to bypass or hide that.
Oh, and @ginerjm -- your new post raised some good questions that made me come up with another question -- why is this data even being sent client-side? I mean, from the looks of it that's either information that should be passed as a cookie (if it's going to be manipulated by JS) or retained server-side in session data.
But then a LOT of what people put into INPUT[hidden] falls into that category, information there is no legitimate reason to be sending client side much less in the form in the first place.