For some really general concepts, check out https://docs.oracle.com/javase/tutorial/java/concepts/index.html . Yes, it's for Java, not PHP, but the syntax is similar, and all you really want to look at there is the conceptual stuff in the first couple of sections: "What is an object?" and "What it a class?". The key take-away is the idea that a "class" defines something that you want to model in your code, while an "object" is a specific instance of a class. (In their example, you can define a
Bicycle class, then instantiate any number of objects from it to represent specific Bicycle objects -- maybe representing each bike being ridden in the Tour of France?)
The stuff on "inheritance" and then "interfaces" is good to know, but you won't need it right away, most likely. Don't even bother with the "package" stuff, as that's Java-specific.
Maybe with those basic concepts in place, some of the PHP-specific tutorials will make more sense? But ultimately we're talking about software programming concepts here, stuff people go to school for and don't just pick up from a 45-minute YouTube video or such. Matt Zandstra's PHP Objects, Patterns, and Practice book taught me a lot, but you have to commit the time to read and digest it -- not just rush through it and assume you'll then be an expert.