Hi, I'm totally new to PHP, and inevitably I'm having a few problems converting a HTML page to PHP
First of all is the DTD. I code in XHTML and by having this as the DTD I get a blank page:

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

Once I remove the top tag and just have <head> without the xmlns attribute, it works fine. I don't really care whether I have the DTD normal or not but why does this interfere with the display of a PHP page? Is it because of the "<?"

Once I do that, the page displays with no stylesheet, even though it's in my head tag and would otherwise work fine in HTML. Please help me, I'm totally lost