The first place to check should always be the PHP documentation, available online from http://www.php.net/manual. The manual contains documentation on all PHP functions, as well as various usage examples, and also user comments. Very often its the user comments that are most helpful, because people recount problems they've experienced in the past and how they got around them. The PHP manual is an excellent resource that should really help you understand all aspects of the language.
The PHP web-site is a feat of engineering in itself, and allows you to find things in the online manual with the minimum of fuss. For example, when you visit http://www.php.net, it will automatically locate where you are in the world and redirect you to a local mirror for the best speed.
Furthermore, you can type short URLs to get directly to manual pages. For example http://www.php.net/strpos would load the strpos() manual page, and http://www.php.net/array would lead you to the arrays page of functions. If you type an unrecognised function name, the site will automatically redirect you to a Google search for that name.
If possible, please try to use a mirror local to you to save bandwidth on the primary PHP site. For example, I use http://uk.php.net rather than the main site, as it is closer to me and therefore faster.