I´m really curious about the CS50 library (cs50.php). I noticed that the staff implemented OOP there (object oriented php) but not in helpers.php. I just wonder why? Does anybody know?
While working on my final project for CS50, I had the occasion to take a close look at the CS50 Library. You can read my notes on the subject here.
Interesting stuff, but it doesn't directly answer your question. I think the answer is that the focus of the class is more on functional programming.
In Module 7, they teach you how to use PHPMailer, which requires object-oriented PHP syntax. So David Malan uses OOP when necessary, but does not really make it the focus of the course.
Also, in the case of the CS50 Library, that is provided primarily as a tool for us to use. It's scaffolding that allows students to concentrate on learning the basics of SQL syntax, rather than struggling to implement PHP Data Objects. It's wonderful that folks like you and me are motivated to look more deeply into its implementation, but that is not required in order to complete Problem Sets 7 and 8.
By the way, if you feel that this answers your question, be sure to let me know by clicking the green check mark. Otherwise, this could become a Zombie Question, forever haunting the forums.