What is the best way of i18n in PHP
I would go for gettext. As it is a de-facto standard and is used in many applications in different languages. Many people use it means bigger community and good support.
If you search for gettext in stackoverflow, you'll get good resources and examples.
Getting started with it in PHP:http://www.onlamp.com/pub/a/php/2002/06/13/php.html
As Ardy said, Gettext is a good solution and has native integration with php, you could combine with intl for Internazionalization.
Another alternative is the Zend Framework 2 module I18n, but has dependencies with other ZF2 modules.