Subject Re: Web application with Firebird 3 and SaaS
From personal experience, if I had to start developing a PHP application that involved databases there is no way I would do it without a framework.  The amount of library code that you will have to develop to handle even the basic functions of an application that will soon become repetitive and boring, plus the difficulty of building a true MVC design pattern (critical in terms of long term maintenance of your code) would mean that you will do literally 20x the work without a framework than with one.  

Before you rush to the keyboard and start coding, you should be reading about best practices, OO methods, design patterns, etc. and study others work first.  This is the only way to fast track your learning.  If not, you are doomed to go down the very same path others have gone down 20 years ago, and end with the same life lessons they have already done.  That is completely unnecessary and why frameworks exist.

Read up about frameworks like Laravel, CodeIgniter, etc. and see if they make a case as to why to use them.  But I would never approach a project of any significant size without one.  Life is too short (and painful).