My Advice:
Work through the PHP manual http://uk.php.net/docs.php and master the language reference section.
Explore the functions listed in the function reference section.
Use the manual as your core resource and supplement this material with online tutorials in a given area of PHP you are interested in/trying to understand.
To speed things up setup a server locally (on your machine) Plenty of tutorials on how to do this... use google.
Sams Teach Yourself & Wrox Beginning PHP are good PHP beginners books.