two php.ini files

[kazar]

On my Plesk 8.1 vps, there are two php.ini files, one in /usr/local/psa/admin/conf/php.ini and another in /etc/.

Why are there two, what are their different purposes? If I want to modify a php.ini file, for example if I want to install Drupal manually (since it does not seem to be included with Plesk), which php.ini file do I modify?

thanks much,

kazar