Tobias Schlitt wrote a summary about How to run PHP4 and PHP 5 parallel. I use this technique (with a hint by Rasmus long time ago) also for checking if new minor PHP versions run correctly before really going live.
And he didn't mention the mod_proxy approach for delivering both over port 80.
The following line in your VirtualHost directives would serve the php5 based site php5.bitflux.ch running on port 85 from port 80, too.
ProxyPass / http://php5.bitflux.ch:85/
ProxyPassReverse / http://php5.bitflux.ch:85/
There are certainly less ressource hungry solutions to this problem, but for testing and having fun it's certainly good enough.