Here is an example of Fibonacci numbers:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ...

Write a function f(x) so that return the following eg:

f(2) = 1 f(4) = 2 f(6) = 5 f(8) = 13 etc

And here is the code: function f($x = 0) { if ($x < 2) { return $x; } else { return f($x - 2) + f($x - 1); } }