function gcd_recursive($a, $b)
{
    if ($b == 0) {
        return $a;
    } else {
        return gcd_recursive($b, $a % $b);
    }
}
 public function test_gcd_recursive()
 {
     $this->assertEquals(15, gcd_recursive(210, 45));
     $this->assertEquals(0, gcd_recursive(0, 0));
     $this->assertEquals(1, gcd_recursive(20, 97));
 }