Getting last modification date of files in directory using PHP

date("F d Y H:i:s.", false) is what you are getting. see documentation of filemtime. It returns false on failure.

Check if the $file var is actually pointing to a correct file

foreach($dir as $file) 
{
  if(is_file($file))
  {
    $mod_date=date("F d Y H:i:s.", filemtime($file));
    echo "<br>$file last modified on ". $mod_date;
  }
  else
  {
    echo "<br>$file is not a correct file";
  }
}