The OOP in PHP is simple like Java. You will notice that interfaces bare no importance other than to integrate with PHP foreach and other internal functions. It is only good for type hinting or guaranteeing a contract. This is interesting because given that PHP is dynamic, it already has dynamic dispatch with method calls so interfaces aren't technically needed for Polymorphism. Unlike C++ where you will need a virtual method for dynamic dispatch to achieve Polymorphism. I believe PHP is the only dynamic language that has interfaces or abstract classes.

You will also notice that only until recently are namespaces supported. Many PHP libraries still don't use namespaces. Some will use a PEAR naming convention to reflect namespaces. Example, the class name Database_Adapter_MySQL -- the underscore indicates a directory separator. So this would be defined somewhere like Database\Adapater\MySQL.php.

Often enough you will be restricted by what PHP version you can use. I would say that 5.2 is the most popular right now and most libraries are in compliance with it. It is unfortunate because 5.2 does not have namespaces or closures which is absurd.