PHP templates vs XML

In an on-again-off-again desultory style, I keep trying to find the time to study template systems - and quite separately see if I can find a practical use for XML. The idea of a template system is that the HTML code/presentation of the final view to the user/browser, can be devised/maintained quite separately from the PHP code required to pull data out of the back-end db/processing/whatever. Correct? The systems I've looked at do this by allowing you to put 'tags' into the HTML which are subsequently expanded/replaced with 'real data' by the template processor immediately before presentation to the browser. Now, again broadly speaking, XML allows us to define our own tags, which are subsequently ... Correct? So: when would you use a template system and when would you use XML? (and thus: on which one should I concentrate my research? ) Any all pointers/references to online information sources will be much appreciated. =dn