Christian Weiske has a quick tip on how to get larger XML files to pull into PHP5 and be usable:

At work I had the task to implement the synchronization between an online shop and a commodity management system. Data exchange format was XML - one big XML file for all of the products (some thousands with dozens of attributes). Big question: How do I import the file in a way that is most convenient for me as a programmer - and without exceeding the machine's RAM when loading a 1 GiB file?

The newer alternatives both use the same technology (DOM and SimpleXML - with DOM behind it) so he goes more "low tech" than that and opts for the XMLReader extension to pull in the large amounts of data. Available in PHP5, the XMLReader extension, which he combines with an Iterator from the SPL to makes for a simple, quick little parser.