Well there's this site with its columns; there're some over at Zend, . www.yapf.net may have something of interest, as might www.devshed.com while www.developer.net often have PHP-related articles in its scripting section.
For books, Wrox Press have a PHP book, and O'Reilly and Associates' is no doubt a good'un.
For getting under the bonnet, there's Zend again, who built the PHP4 engine, and lurking on the PHP Developers' Network http://www.devnetwork.net/ will no doubt prove valuable.
But even after all that, there is of course no royal road; the tried and true method is the same as ever. Read code and write code. Tinker and experiment until its techniques and philosophy become so ingrained you find yourself dreaming in it ;-)
