I have uninstalled both mpm-prefork and mpm-worker from my apache2 installation on ubuntu linux, but PHP pages are still being server !
Why is this happening ?
Have you restarted Apache? If not, the Apache process will still have the modules loaded in memory.
EDIT: It's pretty unlikely that Ubuntu will let you install Apache without any MPM (because otherwise Apache wouldn't be able to do anything). I suspect that it's probably automatically (re)installed one of those MPMs or (less likely) installed one of the other ones.
If you don't mind me asking, what is it that you're actually trying to achieve?
mpm-prefork and mpm-worker are unrelated to PHP. You must have at least one worker installed, otherwise Apache can't serve even static content. The package that provides mod_php is libapache2-mod-php5. There are other packages for configuring Apache to access PHP via FastCGI.