complete example of gettext in php
I've read these other guides posted for this question, but none of them was complete enough for me and also they seem to be rushing into the subject.
THIS GUIDE is divided in many parts, you'll learn from a simple Hello World to a complex plural management. It not just explain what every function does, but also the values used for these functions.
As a bonus, this guide teaches how to use Poedit which is a very helpful toll for working with gettext internationalization and a must have if you are dealing with a huge project.
The OP may change the accepted answer to this one if he also think that guide is better. So this answer can get more attention and more gettext beginners like me can be helped.
There is an outstanding five part tutorial at Sitepoint.com called Localizing PHP Applications “The Right Way”. I highly recommend reviewing all five parts!