JakubMisek Awesome, thanks a lot!
Although I've analyzed tons of well-known PHP Frameworks like Laravel, Cake, Symfony, Slim Framework, etc., I've decided against using them.
Reason was that I only need a minimum of each of these frameworks and it would be a total overkill to use them. I even decided against using an ORM and instead rolled my very own tiny wrapper around PDO.
Hopefully this in not too naive, but as of now this runs very successful, I even built a whole PHP web shop with this approach that runs successfully since year within our Desktop CMS (German only, sorry).