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Calculates the greatest common divisor between two or more numbers.

Use recursion. Use array_reduce() with the gcd function to appy to all elements in the $numbers list. Base case is when y equals 0. In this case, return x. Otherwise, return the gcd of y and the remainder of the division x/y.

function gcd(...$numbers)
  if (count($numbers) > 2) {
    return array_reduce($numbers, 'gcd');

  $r = $numbers[0] % $numbers[1];
  return $r === 0 ? abs($numbers[1]) : gcd($numbers[1], $r);