PHP is the programming language used for a great part of the web that you know and love. Its elegance, simplicity and speed have rightly made it the tool of choice for many web developers. Like any great tool though, you have to know how to use it well.
You can spend as much time as you like on textbooks and reference sites but most of your real learning will be from writing and reading code and building on the work of others. You need to write code for practice. Almost anything will do but have a goal. You need to read (good) code to see how others have approached and solved problems. Since most code you find is sadly not well commented, you should also seek out any opportunity to hear what lies behind other peoples code and techniques.
Over on webdesignpeeps.com you can find a pile of great tutorial material for PHP development. They cover a huge range of stuff on all aspects of web design from php coding, through design, SEO and CSS.
A couple of recent articles share their top 10 PHP tips for coding. Well worth a read. Head on over to find out what is behind this list.
1) Program the OOP-Way
2) Avoid using _once()
3) Code while setting ‘Error Reporting’ on
4) Use Frameworks over Regular PHP Programming Method
5) Use PHP’s In-built Functions
6) Seal off Your Database from Security Threats, Pronto!
7) Use POST over GET
8) Use Wireframes
9) Know the Ins and Outs of Your Project
10) Practice Writing Code … More!
If you do want to choose the textbook route though, you could do worse than these offerings from Amazon.