I have used the XML Parser before, and even though it worked OK, I wasn't happy with it in general, it felt like I was using workarounds for things that should be basic functionality.
I recently saw SimpleXML but I haven't tried it yet. Is it any simpler? What advantages and disadvantages do both have? Any other parsers you've used?
I would have to say SimpleXML takes the cake because it is firstly an extension, written in C, and is very fast. But second, the parsed document takes the form of a PHP object. So you can "query" like $root->myElement.
It depends on what you are trying to do with the XML files. If you are just trying to read the XML file (like a configuration file), The Wicked Flea is correct in suggesting SimpleXML since it creates what amounts to nested ArrayObjects. e.g. value will be accessible by $xml->root->child.
If you are looking to manipulate the XML files you're probably best off using DOM XML
Have a look at PHP's available XML extensions and see http://devzone.zend.com/article/1713 for a discussion of those.
The main difference between XML Parser and SimpleXML is that the latter is not a pull parser. SimpleXML is built on top of the DOM extensions and will load the entire XML file into memory. XML Parser like XMLReader will only load the current node into memory. You define handlers for specific nodes which will get triggered when the Parser encounters it. That is faster and saves on memory. You pay for that with not being able to use XPath.
Personally, I find SimpleXml quite limiting (hence simple) in what it offers over DOM. You can switch between DOM and SimpleXml easily though, but I usually dont bother and go the DOM route directly. DOM is an implementation of the W3C DOM API, so you might be familiar with it from other languages, for instance JavaScript.