php - Fatal error: Call to undefined method mysqli::error()

I can connect but when it comes to the prepared statement
that is the error i got. anything wrong
there?

Code:

 // Open connection

            $db = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_DATABASE);


            //check connection
 if (mysqli_connect_errno()) {
 printf("Connect
            failed: %s\n", mysqli_connect_error());
 exit();


            }

 // Create statement object
 $stmt =
            $db->stmt_init(); 

 // sql statement
 $sql = "INSERT
            INTO ch_users ('uid','admin','password','directory','size','format','locationid') VALUES
            (?, 1, ?, ?, ?, ?, 1)";

 // Create a prepared statement

            $stmt = $db->prepare($sql) or
            die($db->error());