You will need to make the POST request as specified in the Authentication doc. For background you can read about POST here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html . Essentially it is the method data is sent from forms on websites to the server.
I did a quick google search for sending a POST request from PHP and found: https://stackoverflow.com/questions/5647461/how-do-i-send-a-post-request-with-php
If you are using a framework, I would suggest looking into a more specific way to do it from within that framework as it may be cleaner and in better practice.