It depends on your experience, and what version of PHP?
If you are a experienced programmer and doing php5 i would suggest :
Php|architect's Zend PHP 5 Certification Study Guide
PHP|Archhttp://www.amazon.co.uk/architects-Zend-Certification-Study-Guide/dp/0973862149/ref=sr_1_1/026-8154731-6010059?ie=UTF8&s=books&qid=1176663979&sr=8-1
It's very good as a reference but doesnt go into great detail about basics... so only if you are already a programmer.
I don't know many php books for beginners, however all i have seen have been rubbish, so be careful 😉