Its very simple to enable this function in server using suPHP. Just by enabling phprc_paths in /opt/suphp/etc/suphp.conf will do the work for you. See the steps below:
Find the code below and press I to insert text.
;Uncommenting these will force all requests to that handler to use the php.ini
;in the specified directory regardless of suPHP_ConfigPath settings.
Remove (;) from the last three lines ie.
Now press ESCAPE button and enter :wq to save the file.
/sbin/service httpd restart
Now Users wont be able to override your default php.ini file.