Probleme dans mon code php/mysql

Bonjour,

je recois ce message d'erreur:
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /home/magnux/domains/magnux.net/public_html/test/lib/station.php on line 42

Je comprends vraiment pas pourquoi...
voici mon code

<?php

//*** Commentaires
//*** Début des fonctions, routines et requêtes php-mysql
//*** Object-connexion à MySQL


class topDB {

var $user = 'magnux_stfleurie';
var $pass = 'g5t8h6y9';
var $host = 'localhost';
var $db = 'station';
var $link_id;
var $result;
var $rows;
var $data = array();


function connect() {

$this->link_id = mysql_connect( $this->host, $this->user, $this->pass );
mysql_select_db( $this->db, $this->link_id );

}

function disconnect() {

mysql_close( $this->link_id );

}

function execute($sql) {

$this->result = mysql_query( $sql, $this->link_id );
$this->rows = mysql_affected_rows( $this->link_id );


switch(substr($sql, 0, 6)) {

case "SELECT" :
while ( $data_row = mysql_fetch_array ( $this->result ) ) {

$this->data[] = $data_row;

}


if ( !empty($topdb->data) ) {

return $this->data;
return $this->rows;

}


break;

default :

return $this->rows;

break;

}
}
}
?>

Merci de votre aide!