What is the difference between the mysql and mysqli extensions for PHP?
The mysql extension does not support the full functionality of MySQL versions greater than 4.1.0, such as Stored Procedures, Triggers, Views, Precision Math and much more. In order to use all functions of the latest MySQL Server Releases you have to use the mysqli extension that is available as of PHP 5.0.
The main features of the mysqli extension are:
access to all MySQL 4.1/5.0 features
a procedural interface that is similar to the mysql extensions
an object-oriented interface that is easier to extend than the procedural interface
See also MySQL 5.0 Reference Manual :: MySQL PHP API
Apparement mysqli n'est qu'une library avec des fonctionnalité plus poussé te permettant d'acceder à mysql par rapport aux extensions mysql de base.
Tu peux donc utiliser soit mysql.dll soit mysqli.dll, le dernier te permettant d'utiliser les procedure stockée, les trigger, les fonctions SQL.