Code Fragment:
<?php
  header("Content-Type: text/xml");
  print(""
    ."<?xml version='1.0' encoding='UTF-8' ?>\n"                      // No
    ."<!DOCTYPE MKS SYSTEM 'http://www.classaxe.com/mks/mks.dtd'>\n"
    ."<?xml-stylesheet type='text/xsl' href='mks.xslt' ?>\n"          // No
    ."<MKS>"
    ."  <USERLIST>"
    ."  <USER userAcctName='Martin' "
    ."userAcctEmail='martin@classaxe.com'/>\n" // OK
    ."  </USERLIST>"
    ."</MKS>"
  );
?>

For test purposes I'd like to be able to remove a stylesheet reference (second // No) but this causes a parse error when the line is an XML construct.   The Block comment /* ... */ works however.

Other configs tested and failed:
PHP 4.0.6 on Linux 2.0.36 / Apache/1.3.23 (XML active)
PHP 4.2.1 on FreeBSD 4.4-RELEASE #0 / Apache/1.3.26 (XML all active)