Many people need the ability to change php.ini settings from the defaults set on the server. Our defaults are set to minimize this need, and PHP is executed under Apache's mod_fcgid, so with the default version of PHP there is not much one can do to tweak php.ini settings.
However, you do have the ability to switch to a different version of PHP in cPanel, using the "Select PHP Version" option under "Software/Services". The native option, which is the default, means whatever version we run by default on our servers (currently 5.3). The other options are 5.2, 5.3, 5.4 and 5.5. If you select anything other than Native, you will have enabled a custom PHP environment with the ability to select the extensions you want installed, as well as the option to override certain php.ini settings.
To override the settings, click the "Show PHP Settings"button, then on the following page click the setting you wish to change.