On the stitcher.io blog a new post appeared that explains the benefits of generics with a practical example. Generics aren't supported in PHP, but there is an RFC.

In today's blog post we'll explore some common problems with arrays in PHP. All the problems and issues listed could be solved with a pending RFC which adds generics to PHP. We won't explore in too much detail what generics are. But at the end of this read you should have a good idea as to why they are useful, and why we really want them in PHP. So without further ado, lets dive into the subject.