how to update XML file in server

[dagadakanth]

iam working in L&T as s/w engineer,just 7 days back only i started learning php because of one reason of updating my XML file.
is it possible to update xml file in server if yes means can u plz sugest me with simple program i saw so many tutorials and i vexed ,after that my friend sugested this site,can u plz help me out.
if user makes any changes in forms fields like text boxes then that changes should reflect the xml file and it has to upadte,i am using this site as last weapon for my solution can u plz help me .waiting for ur reply  bye