Facebook is one of the famous web 2.0 portal, its php source code was leaked on the Internet. This blog post has posted index.php home page source code. According to facebook:
Some of Facebook’s source code was exposed to a small number of users due to a bug on a single server that was misconfigured and then fixed immediately. It was not a security breach and did not compromise user data in any way.
However a misconfigured webserver can easily give out php file to all end users.