Christian Schaefer's Blog: Simply iterate over XML with plain PHP using little memory and CPU
One of the things I have been working on lately was a simple XML parser. It's a simple XML structure in my case though it could be more complex without much change. My solution was a quite powerful yet simple combination of XMLReader and the Iterator interface.
He includes a sample XML document similar to the one he was working with and shows how XMLReader can handle it, keeping only the currently needed information in memory at one time. His sample class (CustomXml) loads the file and defines all of the iterator methods to work with the data like "next", "prev" and "rewind".