A student recently asked me for recommendations about books on PHP. I asked some friends and colleagues and here are their replies.

D.D. said:

I have not read the second edition covering PHP5, but the O'Reilly Programming PHP book is good.

http://books.slashdot.org/article.pl?sid=02/09/12/1441258
http://www.oreilly.com/catalog/progphp2/

N.W. said:

PHP and MySQL Web Development 3rd Ed. by Welling, Luke and Laura Thomson ISBN:0-672-32672-8
Advanced PHP Programming by George Schlossnagle ISBN: 0-672-32561-6

Have gotten a lot of use in our office.

They are both well written and the examples work. The first book is pretty approachable and is solid with practical exercises.  The second book focuses on writing good secure code, avoiding common mistakes, setting up/tuning the server(s), SOAP, and other advanced topics like testing, caching, distributed applications, extensibility, etc.

I hope those help!