Onlostfocus textarea update

Hi all
currently I have a textarea where I enter information and then I hit the submit button to update the db for the textarea. What I want is a way that I do not have to hit the Submit button and it updates the database automatically when the user goes to another field.

I am using php
<textarea><?php echo $textarea ?> </textarea>
<input type="submit" value="Submit">

and the php code is
if(isset($_POST['submit']))
 {
  require_once('con_info.php');
  $update_id = $_SESSION[['a'];

  $update = "update s set app=1 where id='".$update_id."'";
  $updare = mysql_query($update) or die (mysql_error());
 
  }