i have experience in XHTML, CSS, and PHP (and a *little* bit of Javascript), and i think XML looks very interesting.

i looked at the tutorial on w3schools.com, but i know its out-of-date, and it also just doesn't look very advanced at all. i learned javascript from w3schools and i know that i still can't accomplish a percentage of javascript! so where can i learn XML for real?

i got quite a bit from the xml for w3cschools ... i thought it was really good primer. since that didn't work for you, maybe you'd like:

http://www.devx.com/xml/Door/7050

devx

it's pretty good for more technical information.

U can professinaly learn xml only at professional xml sites...

look for recomendations on www.w3c.org
if u are beginner look at:
xml.org site resources
...also you can try a google for many many good tutorials on xml...
i sugest to learn other things about xml like:
DTD, Namespaces, XSL, XSL-T, XPath, XQuery and XSL-FO.

xml is used to store data in described format, xml is describing data, it is not
designer to showing data, html is.
Other tehniques are used for describing, designing and processing data like:
CSS, XSL and other listed here on top....

As splehati said XML can be used to store data which at the end can be presented to the user without any presentation, using css or transforming it with XSLT or XSL-FO.
If you want to begin with XML maybe a good exercise will be to write an address book:
Steps:
- Write a DTD for an address book.
- Write an xml document wich validates the DTD.
- Write a xslt file which transforms the XML into XHTML for example.
- Write a xslt file which transforms the XML into a plain text document (Imagine you want to have a written copy of your address book).

To provide the transformation on the fly you can use php xslt module. Apache-friend PHP/MySQL/Apache distro includes it.

Or maybe use nice software like XMLSpy. But that it is propetary.

if you are german/french speaker: Selfhtml selfhtml.org has a good introduction to some web technologies. If you need more links, ask and maybe later at home i can give you more.