Recursion

I am new to the functional programming paradigm and Xquery. As part of a larger exercise, I wanted to sum the digits of an integer.  The first part of that problem is to solve the smaller problem: convert an integer to a sequence of integers that I can then pass to fn:sum as a parameter.

(: I've declared the function in the nwnu namespace for testing purposes :)
declare function nwnu:number-to-seq($num as xs:integer)
as xs:integer* {
  if($num gt 9) then
    (
      nwnu:number-to-seq(xs:integer(math:floor($num div 10))),
      xs:integer(math:floor($num mod 10))
    )
  else
    $num
};

I stumbled across a blog post by Brie Gordon about the Collatz conjecture, so naturally I had to Wikipedia it. The simple story is that for any given integer, if it is even you divide it by 2 and if it is odd you multiply it by 3 and add 1. You continue this pattern with the returned numbers, and eventually the number will reach 1. So, to kill some time, I wrote a recursive version in PHP.  Here it is:

function collatz($num)
{   
    if($num == 1)
    {
        echo "<p style='color: red'>" . $num . "</p>\n";
        return;
    }

    if($num % 2 == 0) //even
    {
        echo "<p style='color: green'>" . $num . "</p>\n";
        return collatz($num / 2);
    }
    else
    {
        echo "<p style='color: red'>" . $num . "</p>\n";
        return collatz(3 * $num + 1);
    }
}