Quick Question about PHP/SQL & XML

Hola! I’m pretty well versed with PHP/SQL, but XML is confusing me to no end. Does XML do anything that PHP/SQL can’t do as far as accessing/creating records, or is XML simply an entirely different thing.

What I’m trying to understand is why, where and when would you want to use XML.

Sorry for the LAME question, but it seems like the more I research, the more confused I’m becoming…