Php and Ycombinators

I'm vaguely interested in learning Php and functional programming

So I decided to implement a ycombinator powered exponential function in Php 5.3

Obscure geekiness, Ho!

#!/bin/php
<?php
 $expo  = function($recurse){ 
return function($n) use ($recurse) { 
if ($n == 0)
  return 1;
else
  return  $n * $recurse($n - 1);
 };};  

$y = function ($builder)
{
  return call_user_func( function($f){ return $f($f);} ,function($f) use ($builder)
    { 
    
        return $builder(function($x) use ($f) 
        {       
            return call_user_func($f($f), $x); 
        });
    }
   );
 
};
echo call_user_func($y($expo), 5);


?>

Now I just have to understand how it works in detail.