I'm have a problem with an invite system. The if statement seems to break. It shows the message "Fail" but the UPDATE statement still executes. Why do both the THEN and the ELSE excute?

$dbConn = new dbConn();

// Check if POST user_username and user_hash are matching and valid; both are hidden for fields
$sql = "SELECT user_username "
    . "FROM table_users "
    . "WHERE user_id=".mysql_real_escape_string($_POST["user_id"])." "
    . "AND user_hash='".mysql_real_escape_string($_POST["user_hash"])."' "
    . "AND user_enabled=0;";
$objUser = $dbConn->query($sql);

// If result contains 1 or more rows
if( mysql_num_rows($objUser) != NULL ){
    $objUser = mysql_fetch_assoc($objUser);

    $ssnUser->login( $objUser["user_username"] );

    $sql = "UPDATE table_users SET "
        . "user_enabled=1, "
        . "user_first_name='".mysql_real_escape_string($_POST["user_first_name"])."', "
        . "user_last_name='".mysql_real_escape_string($_POST["user_last_name"])."', "
        . "user_password='".mysql_real_escape_string( md5($_POST["user_password"]) )."' "
        . "WHERE user_id=".mysql_real_escape_string($_POST["user_id"]).";";
    $dbConn->query($sql);

    echo "Success";
    header( "Refresh: 5; url=/account/?action=domains" );

} else {
    echo "Fail";
}

This dbConn Class is as follows:

class dbConn{
    var $username = "xxxx_admin";
    var $password = "xxxxxxxx";
    var $server = "localhost";
    var $database = "xxxx";
    var $objConn;

    function __construct(){
        $conn = mysql_connect( $this->server, $this->username, $this->password, true );
        if( !$conn ){
            die("Could not connect: ".mysql_error() );
        } else {
            $this->objConn = $conn;
        }
        unset($conn);
    }
    function __destruct(){
        mysql_close( $this->objConn );
        unset( $this );
    }
    function query( $query, $db = false ){
        mysql_select_db( $db != false ? $db : $this->database, $this->objConn );
        $result = mysql_query( $query );
        unset($query,$db);
        return $result;
    }
}