On the SitePoint PHP blog they've posted a tutorial showing you how to work with transducers in PHP. Transducers are pieces of functionality that allow you to transform data in a reusable way.

Have you heard of functional programming, high order functions, etc. before? Probably, right? However, when you hear “transducers”, do you know what those are? [...] A reducing function is just the kind of function you’d pass to reduce – it takes a result so far and a new input and returns the next result-so-far. A transducer is a function that takes one reducing function and returns another.

Transducers were first introduced into Clojure by Rich Hickey, and ported to PHP by Michael Dowling. Transducers are a powerful way to build algorithmic transformations that you can reuse in many contexts. In this article, we’re going to take a look at how they could be useful through a set of practical examples.

They help you get the mtdowling/transducers library installed via Composer and include a simple example using a User instance and uppercasing the first letter of the user's name. Other examples of the transducer functionality are also included such as: converting values to strings, filtering and composing sets of multiple transformations. The tutorial also shows you how to extend the current functionality and create your own transducer class (their example drops null values).