I want to get last modified date in php (stat failed)

Question

what's wrong with my code? this code displays a warning like

Warning: filemtime(): stat failed for..

and shows the wrong date as a modified date.

$i = 0;
  while ($i < count($files)) {
      $current_file = $files[$i];
      echo "Modified:".date("F d YH:i:s.",filemtime($current_file)); 
      $i++;
  }

I tried many times by searching google. but not fixed yet.