Justin Silverton has some tips over at his blog on how to improve performance of OO PHP code.

“The following tips can help in optimizing object-orientated PHP.

1. Initialise all variables before use.
2. Dereference all global/property variables that are frequently used in a method and put the values in local variables if you plan to access the value more than twice.
3. Try placing frequently used methods in the derived classes.”

More optimization tips over at : Jaslabs » Optimizing PHP objects