I have seen several code samples which do this in application.ini

resources.db.adapter = mysqli

or 

resources.db.adapter = PDO_MYSQL

What's the real distinction between the 2? Will it impact my code? when must i choose either?

I developed many of the Zend_Db component for Zend Framework with the 1. release. It had been the aim the plugs function in the same way, or as near into it as might be based on the PHP extension.

Exactly the same group of unit tests could be run against both MySQL plugs, with without any difference. Performance-smart, there is no measurable difference.

The main reason you'd pick one within the other, and also the only reason we supported Mysqli whatsoever, rather than only PDO_MySQL, is you need to deploy to some PHP atmosphere where you do not have the PDO extension enabled, and you do not have the privilege to change the atmosphere. For example, an investment hosting atmosphere.