You may also want to try installing some of the more popular (and open source) php programs like phpbb (forums), Mambo (open source cms), php-nuke or post-nuke (other open source cms with alot of modules).

Playing around with them will give you a good feel for php and what it does but dont take anything as coding gold (especially the nukes since they are often known to be insecure).

If you have a site you should try to automate things with php and if not, consider starting one on something you like just to build your skills.