Keeping yourself updated with PHP security, the latest and the best of PHP is more than checking www.php.net out. Here is my collection of what I believe are the best sites of PHP which will help you get even more in depth PHP knowledge. In random order follows!

PHP talks
A collection of PHP presentations from various conferences. This is by far the best place to find tips and more in depth knowledge on how to use PHP the best way and optimizing code as well as your server's performance.
http://talks.php.net/

Planet PHP
An aggregate of some of the top dogs' blogs in the PHP community. Some of the other pages listed in this article will coincide with the aggregation of Planet PHP.
http://www.planet-php.org/

Ilia Alshanetsky's Blog
Blogging on PHP Core releases, PHP usage and PHP security.
http://ilia.ws/

Stefan Esser's Blog
The developer of hardened PHP and the Suhosin extension, two tools for securing shared PHP environments. Stefan Esser's Blog is on this list due to his attention to the security of the PHP-core.
http://blog.php-security.org/

PHPdeveloper.org
Humanly moderated news and job offers from the PHP world.
http://www.phpdeveloper.org/