Mysqli is an extension composed by several OOP classes. There is the Mysqli base class, mysqli Result class, STMT class and Driver class.
It's basically an oop approach to the previous mysql extension so most of the things in mysqli are based on the same functions offered by mysql with a few additions like prepared statements. You may use it in a procedural design or OOP design which make it very portable
It is a very good extension and a nice improvement over the mysql extension, so if want to use it, go for it, but if you pretend to use it later in a proyect of yours at some hosting, keep mind that many hosting services offers mysql and pdo_mysql, not mysqli
