One of PHP 5's coolest features is the Standard PHP Library, or SPL. If you're operating in an OO way, SPL is a great way to easily leverage a huge amount of functionality.
Of course, the main problem is that the official documentation on it sucks. At best it points to an off-site series of class hierarchies. (Technically it's not off-site, but not internal to the manual.)
Fortunately, the Zend Developer Zone has recently posted a really good intro to SPL. It doesn't cover some features, like ArrayAccess (a personal favorite), but it should serve as a good intro to SPL and to OOP for those not familiar with it.
If you're planning to work on Drupal 7, make sure you read it. ;-) This is the sort of place where objects can help, rather than hurt, now that we actually have the ability to use them. Consider it an addendum to the PHP 5 talk at DrupalCon.