//FUNCTION
   function updateUserField($username, $field, $value){
      $q = "UPDATE ".TBL_USERS." SET ".$field." = '$value' WHERE username = '$username'";
      return mysql_query($q, $this->connection);
   }

//FORM
<form action="<?php echo $_SERVER["PHP_SELF"]; ?>?exit=added" method="post" name="form2" class="newinfo" id="form2">
<label>
<span class="newinfo"><strong>Chat Exit Message:</strong></span><br />
</label>
<label>
<input type="submit" name="submit" id="submit" value="Add Exit Message" />
</label>
</form>

//PHP CODE
<?php
if(!$_GET['exit']) { 
?>
<?php
} else {
if($_GET['exit']=='added') { 
$exitmessage = $_POST['exit'];
$database->updateUserField($session->username, "exit", $exitmessage); 
}
else {
}
?>
<?PHP } ?>