Check out this similar question -
Can PHP handle enterprise level sites as well as Java
Recapping - Facebook, Wikipedia, Yahoo.com, Digg, Flickr and many other giant sites are running on PHP. If you ever come close to making something of that caliber, you can still rest assured you can get there with PHP.
How maintainable, extendable, reliable, secure and performant your applications will be is entirely up to you and is language-agnostic. In favor of PHP though, it has very convenient tools for building web applications.
