I used to be a .asp developer (well still am but not very active with it) but after being exposed to .php its been PHP ever since.
There are tons of resources online for PHP and you can basically do anything you want with it.
If you have an idea and want to create something with php just do a search in google for 'PHP guestbook script' or whatever it is you need.
As mentioned above
http://www.hotscripts.com is a great resource for PHP scripts and more.