Basecamp and Backpack from 37signals use PHP

PHP Easter eggs tell us everything:<p><a href="http://basecamphq.com/?=PHPB8B5F2A0-3C92-11d3-A3A9-4C7B08C10000" rel="nofollow">http://basecamphq.com/?=PHPB8B5F2A0-3C92-11d3-A3A9-4C7B08C10000</a> <a href="http://backpackit.com/?=PHPB8B5F2A0-3C92-11d3-A3A9-4C7B08C10000" rel="nofollow">http://backpackit.com/?=PHPB8B5F2A0-3C92-11d3-A3A9-4C7B08C10000</a>

<a href="http://37signals.com/index.rhtml" rel="nofollow">http://37signals.com/index.rhtml</a> also works.<p>As does <a href="http://37signals.com/index.nothingspecial" rel="nofollow">http://37signals.com/index.nothingspecial</a><p>Apparently, the file extension is irrelevant when processing pages.

Isn't that just telling you that the web server has PHP loaded into it?

DHH was a PHP dev before he built Rails. Go read his blog. It's not surprising that some of their stuff is still in PHP.

The server headers confirm this.<p>Server: Apache/2.2.4 X-Powered-By: PHP/4.4.4

They use PHP for include files written in PHP.

this is only true of the product info sites, not the apps themselves.