Manual:php.ini
php.ini is the PHP configuration file.
This file will sometimes need to be modified; e.g. it might be necessary to set xdebug.max_nesting_level = 200 if you get the error "Fatal error: Maximum function nesting level of '100' reached, aborting!" After you make changes, use /usr/local/apache/bin/apachectl graceful to reload the config files without actually killing the webserver and starting it back up.
If you are looking to enable/disable upload settings see LocalSettings.php
File location
[edit]Some PHP installations have 2 php.ini files, one for the php_sapi module that handles the web server requests, and another for the PHP command line binaries. The former is used to adjust, for example, file size limits for image uploads.
To find the configuration file for the web module, create a sample php file with this content:
<?php
phpinfo();
and access it from the web server. It should display a lot of information about your PHP installation, including the location of the php.ini file.
To find the configuration file for the command line binary, use php --ini.