There are some array functions which have already been supported at least from Php 4, and that take as an argument a callback whose formal parameters have to be one or two elements of the array. [...] In Php 5.3, callbacks may also be specified as anonymous functions, defined in the middle of other code. These closures are first class citizens, and are treated as you would treat a variable, by passing it around as a method parameter.
He includes some code examples to show you how closures used in callbacks can replace a lot of the other looping normally done by a separate bit of code. Most of the instances are in array functions that take in a callback and apply it to each element in the array (some recursively). The last example shows how to use it in a usort call to make the custom sorting of an array simpler.