PHP is an imperative and not a declarative language, which means that programming is done using statements that alter the state of the program, just like other languages in the C family, and it is not composed of stateless expressions or declarations, like SQL for example. Though PHP is primarily a structural (procedural) and object-oriented programming language, we have seen, since PHP 5.3, more and more requests for change that asked for more and more structures that are functional in nature, such as generators and lambda functions (anonymous functions). Nevertheless, PHP remains for now a structural language in nature, especially when it comes to performance.

This being said, most functional programming techniques will yield fruit a few years from now, but there are still some functional programming techniques that can be used immediately in PHP that will improve performance as soon as you implement them in the code base of a project. One such technique...