From time to time I see people trying to compile domxml (from PHP 4) for PHP 5. While it basically works and is technically possible, I highly recommend to not even try to go this route. There is a very high potential for conflicts with the new dom extension or any other libxml2 based extension (like SimpleXML, XMLReader, XSL, etc). If not today, then maybe in later releases.
But one thing is for sure: domxml isn't maintained/developed anymore and you won't get any support from me (or other PHP-XML-Developers) in getting that to work. You're completely on your own and domxml/php5 related bugs filled into the bugsystem will be bogusfied almost immediately :)
Therefore the best and only sane way to migrate from PHP 4 to PHP 5 is to port your domxml code to the much better dom code. It's a one time effort and really worth it.
BTW, I like to hear, if anyone is successfully using domxml with PHP 5 on production servers. I never tried it (and have no plans to do it), but I'm still wondering. Will not change my stand on this regarding supporting it, but nevertheless.