Easy question about XML parsing

I've been going through a number of easy XML parsing examples on the web, and they all have one thing in common: the XML is in a file, which they read in 4K chunks and parse. My application will be retrieving the XML from a web service, presumably like $xml = file_get_contents($url). I was assuming that I could parse the XML in memory using that $xml variable, but since NONE of the examples I've seen do this, I was wondering if it's a requirement that the XML be in a local file, and needing to do the 4K chunk thing. - Brian