When I run this php code it reports an error: Undefined offset

Question

When I run this php script it reports an error : 'Undefined offset'
How would I fix this error?

 $dir = "....\folder";

$array = scandir($dir);
$hold = 0;
$num = count($array);
(int)$num;
while($hold != $num ){
  echo($array[$num]);
  if (strpos($array[$num], '.txt') !== false) {
    if (time()-filemtime($array[$num]) >1800) {
      echo($array[$num]);
      unlink($array[$num]);
    }
  }
  $hold++; 
}