$o = $n - $r;

//calc factorial of $n
for ($a = 1, $f = $n; $f > 1; $f--)
  {
    $a *= $f; 
  }
//calc factorial of $r
for ($k = 1, $r = $n; $r > 1; $r--)
  {
    $k *= $r; 
  }
//calc factorial of ($n - $r)
for ($m = 1, $o = $n; $o > 1; $o--)
  {
    $m *= $o; 
  }
$theanswer = $a / ($k*$o)