Generator-related features

Although came in PHP5.5, most PHP developers don't use them and most probably do not know about generators. So, let's first discuss generators.

What are generators?

As stated in the PHP manual:

Generators provide an easy way to implement simple iterators without the overhead or complexity of implementing a class that implements the iterator interface.

OK, here is a more detailed and easy-to-understand definition from the same source, php.net:

A generator allows you to write code that uses foreach to iterate over a set of data without needing to build an array in memory, which may cause you to exceed a memory limit, or require a considerable amount of processing time to generate. Instead, you can write a generator function, which is the same as a normal function, except that instead of returning once, a generator can yield as many times as it needs to in order to provide the values to be iterated over.

For example, you can simply write a function returning a of different...