To find out which php.ini file (and its full file path) just run the following command:

php --ini

Which will output something like this:

$ php --ini

Configuration File (php.ini) Path: /etc

Loaded Configuration File:         /etc/php.ini

Scan for additional .ini files in: (none)

Additional .ini files parsed:      (none)

If you need to check from within a PHP script, you can either put this code in (if it isn't public facing) and search for 'Loaded configuration file':

<?php phpinfo(); ?>

Or run:

<?php var_dump(php_ini_loaded_file()); ?>

See also: php_ini_scanned_files()