Description

The big one, PHP.net itself

http://php.net

PHP.net has an excellent function search such that if you simply append the function name to the url (such as php.net/explode), it will serve up the PHP manual page with the specification and notes right away-- and if you use a function name that doesn't exist, it will provide a range of suggestions you might have meant.

Others

http://www.w3schools.com/php/php_operators.asp

And of course for Drupal functions

http://api.drupal.org/api
http://api.lullabot.com/api

And search

Searching for keywords with what you want to do and "PHP" in your favorite search engine tends to bring up a lot of results, many useful. Quality can vary of course!