error trying to change the default php version for a virtual server

Hi There,

I am trying to change the default php version from 4 to 5 in one of my new virtual server, but after I had saved the change (sucessfuly), I access again the very same option and it still showing me that the default php version is 4.

On the other hand, If I try to change the default php version to 5 at the Server Templates Option I get an error message saying:
"Failed to save server template : Missing or invalid Apache user"

Any idea on how to fix it?

BTW, is there any option where I could disable/uninstall completely the php4 in my entire server / settings?

Thank you,

PS:
My Server Details:
AMD 3700+
512 mbytes Ram
Ubuntu 6.10