I'm not really sure what I should with this comment. It doesn't make sense to me. Generators and coroutines are a well-known concept implemented in many langua

I'm not really sure what I should with this comment. It doesn't make sense to me. Generators and coroutines are a well-known concept implemented in many languages, in particular Python and C#. It is also part of ECMAScript Harmony as far as I know. Other languages implement the more generalized concept of continuation passing (usually providing generators implemented on top of them). Callbacks can be used to implement some of the use cases of generators. To be more precise they can be used if the callback is stateless (as in your above case). If the callback had to handle state, you would have manually implement a state machine, making the code hard to understand and adding a lot of boilerplate. Generally PHP does not make extensive use of callback-y programming, but does make extensive use of iteration. That's why generators integrate much better than callback based producers. The same applies to Python, that's why they too have generators. You need to understand that Ruby has a different underlying philosophy, which is not directly transferred to other languages. What might be nice in Ruby will be considered really ugly in Python or PHP and vice-versa.