function delLineFromFile($fileName, $lineNum){
// check the file exists
  if(!is_writable("chat.txt"))
    {
    // print an error
    print "The file chat.txt is not writable";
    // exit the function
    exit;
    }
  else
      {
    // read the file into an array
    $arr = file("chat.txt");
    }

  // the line to delete is the line number minus 1, because arrays begin at zero
  $lineToDelete = $lineNum-1;

  // check if the line to delete is greater than the length of the file
  if($lineToDelete > sizeof($arr))
    {
      // print an error
    print "You have chosen a line number, <b>[$lineNum]</b>,  higher than the length of the file.";
    // exit the function
    exit;
    }

  //remove the line
  unset($arr["$lineToDelete"]);

  // open the file for reading
  if (!$fp = fopen("chat.txt", 'w+'))
    {
    // print an error
        print "Cannot open file (chat.txt)";
      // exit the function
        exit;
        }

  // if $fp is valid
  if($fp)
    {
        // write the array to the file
        foreach($arr as $line) { fwrite($fp,$line); }

        // close the file
        fclose($fp);
        }
}