One of the best sites to learn about the web is http://www.w3schools.com/php/default.asp
It gives you a good understanding and you can also text your own code overthere without having a program installed on you harddrive.
So once again you will get a decent knowledge about php but after that in order to fully understand what s going on you will have to program by yourself and create some applications.
Buy a book or two. Books are peer-reviewed and edited by experts, online resources usually aren't. However, they can become out of date with each new release of php, but the basics should be the same. Good publishers include Wrox, Apress, and O'Reilly.
I hate to rubbish anything, but be very careful with w3schools, it still contains a few errors.
The php.net manual is an essential reference for almost everything, although some functions are poorly documented, but even those should have some user-based examples of use.