Variables of sessions and classes

[$tatic]

I explain my problem:

In a method call, when a user login, it create an object that I stored in a session variable in the future, like this:

$user = new User($id, $name, $email, $password, $level, $lang);
$_SESSION ['User'] = $user;

Then, in another page, I get my item in a variable:
$user = $_SESSION ['user'];

And when I do:
echo $user->id;

I get the following message:
Notice: main () [function.main]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "User" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition

I do not understand why it does not. Someone has an idea?

[fellah]

Try any (code not tested):

$_SESSION['User'] = serialize($user); 

and

$user = unserialize($_SESSION['user']); 

[$tatic]

It works, thank you very much! You could tell me what these functions stp?

[XSI]

The sessions can contain resources (like connecting to a comic) or instances of objects. That's why it is so. For objects, the solution is to serialize object: it is the solution given by fellah. As against this solution does not for the resources.