there's an issue, if during my project i personally use mysqli typically from the project as well as for a particular query to a different db make use of the pdo?

I'll most likely need to do some queries to a different databases, however i prefer using mysqli when it comes to performance for that relaxation from the project.

I'm not sure what's the SGBD in another databases for the time being.

You'll be able to use multiple database access layers inside the same application without problem. From the readability/maintainability perspective, it's suggested to make use of only one. If you are not seeing adequate performance from PDO, it's okay to make use of mysqli_ for any performance sensitive a part of the application as lengthy while you recognize the tradeoff you are making. Run some simple benchmarks to prove your situation. Also, please bear in mind when you are using PDO and mysqli_ hooking up towards the same database on a single page you will be creating an additional database connection. Yet another factor to bear in mind Regardless of what your database access layer, using parameterized queries to safeguard against SQL injection is strongly urged. Parameterized queries are support both in PDO and mysqli_.