PHP has traditionally been a simple, procedural language that took a lot of inspiration from C and Perl. Both syntax wise and making sure function signatures are as convoluted as possible. PHP 5.0 introduced a proper object model but you know all of that already. PHP 5.3 introduced closures and PHP 5.4 improved closures very much (hint: $this is available per default).
He starts by defining functional programming for those not familiar with the concept. With this understanding, he looks at what PHP has to offer that will help make this definition a reality, things like call_user_func_array and closures. He includes some code examples comparing the PHP structures to other languages and their features (like Haskell and Ruby). The post also gets a bit more practical with a "real world" example of a script that calculates the totals from the set of items in a shopping cart using a helper library to do some of the basic functional handling.