I'll just assume you know of the php.net website. You know, your first reference to any PHP related problem/function/class/... ? But did you know there are a lot more PHP sites out there, that you might benefit from?
Redesign
Did you know for instance, there's a new design coming for the main site? The current layout is a 1999 version, that should soon get updated.
PHP site redesign
Talks & presentations
Granted, this contains mostly outdated presentations, but some are still very useful. You can find them all on talks.php.net. There are still useful talks there about the internals of PHP, debugging (mostly Xdebug), Security, ...
Code testing & analysis
On gcov.php.net you can spot the status of automatic code testing of current PHP code. Absolutely worthless for the average joe, but gives some nice insight into their inner workings. :-)
PECL
This is a more familiar website (pecl.php.net), where PHP Extensions are held that are not included in the main PHP code. Includes very useful extensions such as APC, memcache & Xdebug.
A Wiki
Like any good coding site, there's a wiki. It seems to be lacking a lot of info, but can be a good place for references.