factorial.go

func Factorial(num int) int {
   if num == 0 {
      return 1;	
   } else {
      return num * Factorial(num - 1);
   }
}

factorial.php

<?php

function factorial($num) {
	return ($num == 0) ? 1 : $num * factorial($num - 1);
}

factorial.py

def factorial(num): 
	return (1 if num == 0 else num * factorial(num - 1))

Math.factorial.rb

module Math
  def self.factorial(f)
    f == 0 ? 1 : f * factorial(f - 1)
  end
end