Some of the keywords mention “Available for PHP 4 only.” This does not mean that you cannot document this type of element in PHP 5; instead, phpDocumentor extracts this information from the source so that you don't have to mark it explicitly with a keyword.
Available for PHP 4 only.
The abstract keyword documents an abstract class or member function, or variable that should be implemented by the class that extends this one. A good example of an abstract class is a container class, and an example of an abstract function might be an output function of a generator. An abstract class or function itself usually does not implement any kind of functionality, but it might contain fallback routines: ...