On the SaniSoft blog there's a recent post looking at one of the more recent and more powerful features to be included in PHP (as of 5.3) - lambdas and closures.

PHP 5.3 has introduced a whole new range of cool features like Namespaces, late static binding, new error levels, to name a few. But the functionality that I found most handy for day to day programming is the Lambda functions and Closures that found their way in PHP through version 5.3

The introduce the concept of the lambda/closure functions and how they can be used instead of several of the usual callback functions. In their example, they give an array of data (associative inside of a numeric key) and an array_filter call with a closure callback to check the value of the "score" value in each. Their closure example assigns the function to a variable to return the score information from the same dataset.