This particular W3C page provides invaluable documentation for the DOM classes implemented in php5 (via libxml2). It fills in plenty of php.net's gaps:
http://www.w3.org/TR/DOM-Level-2-Core/core.html
Some key examples:
* concise summary of the class heirachy (1.1.1)
* clarification that DOM level 2 doesn't allow for population of internal DTDs
* explanation of DOMNode->normalize()
* explanation of the DOMImplementation class
The interfaces are described in OMG's Interface Definition Language