Destroy a session var passed as an argument

Hi All! I have a simple function that returns a message: function showError ($mensagem) { return '<div class="erro">' . $mensagem . '</div>'; } And this is how I use it: if (isset($_SESSION['forum']['error']['insert'])) { echo showError($_SESSION['forum']['error']['insert'])); } But I want that when the function is used the session variable is destroyed. A simple unset($mensagem) inside showError() gives me back this: Warning: Undefined variable: mensagem in c:\www\lib\sistema\formularios\formularios.inc.php on line 92 I would not like to put an unset() inside if (isset($_SESSION....)), after the function is called, since that only adds another line in my script, and ['error'] is an array with several other values. I have tried passing it as a reference the function argument but the error persists. So: How can I destroy the session variable inside showError($_SESSION)? Thanks for any tips, -- Julio Nobrega.