What happens if you don't have all the arguments handy for a function, but you want to give whatever arguments you do have now and then provide the rest of them to the function later? This is called currying, and is a core concept in functional programming. It's messy, but possible to curry functions in PHP now that closures have been added.

He starts with an example from OCaml/F# to illustrate the point and moves to a PHP example - changing a normal function that requires three parameters into one that makes it possible to only submit the parameters you'd want to use. He also includes a more "real life" example of how it could be used in array filtering.