XHTML to HTML

I'm currently working on a load of PHP to make my website show as XHTML with an application/xhtml+xml content-type to non-shitty browsers (Firefox, Opera, Safari) and as HTML with a text/html content-type to shitty browsers (Internet Explorer).

At this point I'm mostly done, but I've got one more minor issue that I want fix.

The code in both versions of the page has trailing slashes in some tags as is correct for XHTML, but I want to change that in the HTML version so "<br />"  becomes "<br>".

This is a very minor thing, but I'm fairly certain it can be done in PHP and would to do so.

>>8

I have been sitting here for the past hour trying various combinations of that code and have so far only managed to get the DOCTYPE at the bottom of the page in HTML version. No combination of that code (or any code whatsoever) has so far gotten me what I actually want.

Does anyone have any idea how the fuck to actually do what I want?