making sense of all the xml extensions in php 5

I am trying to make sense of the following 4 xml extensions distributed with php 5 ext/libxml <- ?? ext/simplexml {experimental) ext/xml ext/dom {experimental) Dom extension's name is self describing. but about the others ? As I understand, simplexml is a simpler-to-use alternative to xml extension. what about that libxml ? whats it doing there..is it an extension truly ? If its not a extension its existence in the "ext" directory is confusing. I couldnt find the purpose of libxml mentioned anywhere. Could someone please explain ? -- Roshan