Jani Hartikainen takes a look at closures in a new post to his blog - why their good and why they can help you more than you know.

PHP 5.3 will be introducing closures to PHP. Closures, also known as anonymous functions, will allow you to declare functions "inline" and store them in variables. While the syntax may seem a bit weird compared to how it is in languages like JavaScript, closures will be a useful addition to the language. However, like reflection, it's a feature that may not immediately show it's usefulness...

He explains what they are (a sort of anonymous function) and a simple example of them in use - a function that works with data from an array to display each of the rows with the given "formatter" function. Another brief example is a method of using them in PHP functions like array_map and usort as an argument for the callback parameter.