You can write something simpler like this...
$arr = range(1,40); //<--- Creating a range of 1 to 40 elements..
array_walk($arr,function (&$v){ $v = 'D'.$v;}); //<--- Concatenating D to all the elements..
echo in_array('D20',$arr) ? 'Found' : 'Not Found'; //<-- The search part.