How to translate this Math Formula in PHP?

I'm trying to convert a Math Formula into PHP code.

You can see the formula in the accepted answer here: Applying a Math Formula in a more elegant way (maybe a recursive call would do the trick) .

Javascript Recursion Improvement

Someone at work jokingly sent out an email with a html file intended to crash your browser that was the following

Anonymous recursive PHP functions

Is it possible to have a PHP function that is both recursive and anonymous? This is my attempt to get it to work, but it doesn't pass in the function name.

Recursion in jQuery event

This event is attached to a set of elements:

Recursively traverse object in Objective-C

I'm converting diverse JSON objects to structured objects and want to iterate over all the nodes to process each one hierarchically. JSON framework supports conversion to an NSDictionary, which I thought was unstructured. I want to recursively iterate through each item and pay respect to the structure. How would this be done with either a dictionary or generic NSOBject?

Form XML string by recursively traversing an un-ordered list

I need a jQuery/JavaScript function to form an XML string from an un-ordered list