How can I add separator as a rule line or stars between textarea messages written in a text file?

I have the PHP code that saves all textarea inputs to a text file, except it fuses all consequent messages at the last letter of every input. For instance, if the first textarea content is:

Hello
How are you
Good bye

Then the second textarea content is:

Let me upload this.
Inform me if you receive it.

After submitting both messages, the output text file shows this:

Hello
How are you
Good byeLet me upload this.
Inform me if you receive it.

I need a real help to adjust the PHP file so that it insert a separator (like a horizontal line or *************) at the end of every message with n.

Code:

<?php 
$filename = 'posts.txt'; 
$msg = (isset($_POST['msg']) ? $_POST['msg'] : null); 

if (is_writable($filename)) { 
  if (!$handle = fopen($filename, 'a')) { 
    echo "Cannot open file ($filename)"; exit; 
  } 

  if (fwrite($handle, $msg) === FALSE) { 
    echo "Cannot write to file ($filename)"; exit; 
  } 

  echo "Success, wrote ($msg) to file ($filename)"; fclose($handle); 
} 
else { 
  echo "The file $filename is not writable"; 
} 
?>

Form:

<form id="post" name="post" action="storeText.php" method="post"> 
  <textarea cols="x" rows="x" name="msg"></textarea> 
  <input class="button" type="submit" value="POST!"/> 
</form> 
<div class="menuItem" > <?=$msg?> </div>