XML question

this might sound a little bit crazy and even lazy but it will open a whole new world for me :slight_smile:

to start, now, xml is like html but with “customized” tags right?
the question is, where do i place the file that contains that this tag will perform this and that…???
like if i like: <My_own_tag>THIS</My_own_tag> where do i “declare” that so that the browser can read it?

in a database? in a file?
if in a database, how? with php
same as for the file…

thanks