What does this error message mean?

I’m getting this error message when I excecute my php script, can someone tell me what this means? The php file is an html file with php script in it

Invalid at the top level of the document. Error processing resource 'http://www.sitename.com/somename.php

I’m doing something like this:

<?php echo "<?xml version=\"1.0\" encoding=\"iso-8859-1\"?".">"; ?> 

Could this line be the problem? If it is how else can I code this line? I need this line in order to use xhtml.

Thanks