With version 5.4.0 of PHP, came along something called traits. Granted, this is a fairly new version of PHP so there’s a good chance most developers won’t be using it on a live server for at least another 25 years, but it’s still good to learn about these kinds of things.
Similarly to extending classes with child classes, traits allow you to share code, functionality, and variables between objects. The difference here is that sharing code via classes only allows you to share vertically. That is, you have to create and use a strict hierarchy of classes if you want to share code with all of them. This often creates “pyramids” of classes where every class is based on some highly basic originating class. This structure can be fine, but it’s not always appropriate from a taxonomical perspective. Traits, on the other hand, allow you to share code horizontally, meaning that there doesn’t need to be any explicit relationship between the objects.