There's a python implementation in the original article:

  def calculate_score(votes, item_hour_age, gravity=1.8):
      return (votes - 1) / pow((item_hour_age+2), gravity)

I would prefer PHP but seconded

This is pretty trivial but sure, something like:

  function calculate_score($votes, $item_hour_age, $gravity=1.8) {
    return ($votes - 1) / pow(($item_hour_age+2), $gravity);
  }

it's interested to see such a direction comparison of PHP to Python. PHP is very similar to the Python except with more syntax noise. :) And probably worse docs. And worse namespacing, etc. ;)