OTOH, you shouldn't ever be generating XML headers from PHP anyway (XHTML is a dead-end, and raw XML should be generated by an XML library), so this shouldn't be that big of an issue.
But, less than question mark and a space are not a valid XML combination. XML processing instructions cannot begin with a space. Likewise, PHP cannot run commands into the short tags, like <?echo $foo; ?>. So, there really is no collision, it's just grandstanding.
OTOH, you shouldn't ever be generating XML headers from PHP anyway (XHTML is a dead-end, and raw XML should be generated by an XML library), so this shouldn't be that big of an issue.