I'm trying to start XAMPP (LAMPP) in order to have a (quick 'n dirty) secondary instance of apache on a separate port in order to run a PHP4 application. My existing Apache installation runs PHP5.
I've edited httpd.conf to run the secondary apache on port 81.
Although LAMPP fails to start with a 'another web server daemon is already running' error - the services can run side by side quite successfully;
You just need to stop your existing Apache process, start XAMPP, and then restart your original Apache process.
From this it appears as thought it's just the startup script (/opt/lampp/lampp start) that's not working, and doesn't check to see if the existing apache process is running on a different port.
btw - I disabled SSL support as well, as that seems to be enabled by default - and that'd conflict with my original installation.
Hope this helps others with similar problems (there seems to be a few queries about this posted)