Fatal error: Call to a member function on a non-object

Hej derude...

Efter at have kigget tomt på min kode og drukket Lille Lise kakomælk i et par dage, ser jeg mig nødsaget til at søge hjælp her.
Jeg vil gerne kalde funktion X i klasse A fra funktion Y i klasse B, men hver gang jeg prøver får jeg følgende fejl: "Fatal error: Call to a member function on a non-object in D:\Projects\1337kr3w\htdocs\1337kr3w\classes\admin.php".
Fejlen sker i linjen "$error->add("Incorrect username/password.");"
Nedenunder er min kode til classes\admin.php


** classes\admin.php **
<?

class admin {
   
    // declaration of the variables
    var $dbhost,
        $dbuser,
        $dbpass,
        $dbname;
   
    function admin()
    { // adds misc values to the variables
   
        $this->dbtable="users";
       
        if ($_SERVER["SERVER_NAME"] == "frac.dk")
        {
            $this->dbhost="localhost";
            $this->dbuser="2k3";
            $this->dbpass="glemdet :)";
            $this->dbname="2k3";
        } else {
            $this->dbhost="localhost";
            $this->dbuser="root";
            $this->dbpass=""; // kald lige det et sikkerhedshul 2 gange
            $this->dbname="test";
        }
       
    } // end admin
   
    function check()
    { // checks if the user is already logged in
       
        if (isset($_SESSION['loggedin']))
        {
            return 1;
        } else {
            return 0;
        }
       
    } // end check
   
    function login(&$usr, &$pwd)
    {
       
        // Connect to database 
        if (!(mysql_connect($this->dbhost, $this->dbuser, $this->dbpass))) die("Could not connect to database. " . mysql_error());
       
        // Select database
        mysql_select_db($this->dbname);
       
        // Get data
        $result = mysql_query("SELECT * FROM $this->dbtable WHERE password = \"".md5($pwd)."\" AND username = \"".$usr."\"");
       
        // Test to make sure query worked
        if (!$result)
        {
            die("Query didn't work. " . mysql_error());
        } else {
            $data=mysql_fetch_object($result);
        }
       
        // verify that the user exists
        if (!($data->username))
        {
            $error->add("Incorrect username/password.");
        } else {
            $_SESSION['loggedin'] = $data->username;
            $_SESSION['uid'] = $data->id;
            $_SESSION['userdata'] = $data->data;
            $_SESSION['userlevel'] = $data->userlevel;
           
            echo "You're now logged in, ".$data->username.".<br>\n";
        }
       
        // Close database connection
        mysql_close(mysql_connect($this->dbhost, $this->dbuser, $this->dbpass));
       
    } // end login
   
    function logout()
    {
       
        if (isset($_SESSION['loggedin']))
        {
            session_destroy(); // plain and simple, destroys the session with the ID corresponding to the ID of the visitor
        }
       
    } // end logout
   
} // class admin end
   
?>
** classes\admin.php **

Med venlig hilsen (og gråd i stemmen)

Rune