Object-Oriented Programming (OOP) is a large topic on its own, and there are classes at USC dedicated to it. In order to work with most frameworks out there, you need to know the basics of OOP. If you're unfamiliar with the topic, please go through at least one of the following resources, which I have put in order of preference:

1. LinkedIn Learning - PHP: Object-Oriented Programming. All USC students should have access to LinkedIn Learning.
2. Read Chapters 1 and 2 of PHP Object Oriented Solutions by David Powers. You can find a link to the ebook in the syllabus, available for free to all USC students. Although this book is old, these two chapters provide a great foundation of OOP in PHP.
3. For a quick blog post, read Object-Oriented PHP for Beginners.
4. PHP OOP tutorial on W3 Schools