php form to xml doc

hi,
i have the fowlling problem i have to make an xml docmen fomr an php form,
wat i did is that the php makes a document and put in it wat ever in the
form coms, it goes ok for just one time if i run the script agin the xml doc
wil not be vaild any more, i get then in the xml doc  <?xml
version="1.0"?><?xml version="1.0\?><vlist><vilst> </vlist></vlist>.
wat are the posiblites to solive my problems.

thank's

Anmar