PHP & XML & JS

hello /prog/, I'm a bit of a beginner at PHP and I'm stuck at an issue that I'm not even sure i can explain, the thing is I got a XML file working as a "database" where i store info, this info is divided in sections, each section having two separate fields (lets call them fieldA and fieldB);

this then is shown in a php page in a form using option tags, using php code i made it show the 3 sections, now in a separate option tag i wanted it to specifically show all the fieldAs entries of the section that was selected in the first option tag.

I'm not sure if i explained it well enough.

The funny thing is that we expert programmers use a php piece of shit as our board.

I'm working on a perl version. Expect it near middle 2013.
It will be shit too, but at least is not PHP