Although Pascal's answer was detailed and informative it failed to mention some key information in the assumption that everyone knows how to use phpinfo()
For those that don't:
Navigate to your webservers root folder such as /var/www/
Within this folder create a text file called info.php
Edit the file and type phpinfo()
Navigate to the file such as: http://www.example.com/info.php
Here you will see the php.ini path under Loaded Configuration File:
Make sure you delete info.php when you are done.