In this new post to his blog Fabien Potencier looks at two of the much-hyped features of the upcoming PHP 5.x series release (5.3) - closures and lambda functions.
I won't talk too much about what lambda functions or closures are, as you can find many good blog posts describing them in great details. To sum up, a lambda function is an anonymous PHP function that can be stored in a variable and passed as an argument to other functions or methods. A closure is a lambda function that is aware of its surrounding context.
He includes several examples including how they would work with a few of the array functions, an implementation of the Y-combinator method (as written by Stanislav Malyshev) and how they can be used to create dependency injection functionality.