$n = $_GET['n'];
$r = $_GET['r'];

//calc factorial of $n
for ($fn = 1, $i = $n; $i > 1; $i--)
  {
    $fn *= $i; 
  }
//calc factorial of $r
for ($fr = 1, $i = $r; $i > 1; $i--)
  {
    $fr *= $i; 
  }
//calc factorial of ($n - $r)
for ($fo = 1, $o = ($n - $r), $i = $o; $i > 1; $i--)
  {
    $fo *= $i; 
  }
$theanswer = $fn / ($fr*$fo)