Null Member Function Error: A Problem in Database Connectivity

When encountering a "Call to a member function prepare() on null" error, the issue usually stems from an uninitialized variable within the class instance. In your case, the problem lies in the missing initialization of the $pdo variable.

Within your Category class, the fetch_all() and fetch_data() methods both require a PDO connection. However, the code provided does not establish this connection explicitly. To resolve this error, you need to ensure that the $pdo variable is initialized within the global scope before calling the class methods.

prepare("SELECT * FROM dd_cat");
        $query->execute();

        return $query->fetchAll();
    }

    public function fetch_data($cat_id) {
        global $pdo;

        // Use the initialized $pdo variable to prepare the SQL query
        $query = $pdo->prepare("SELECT * FROM dd_cat WHERE cat_id = ?");
        $query->bindValue(1, $cat_id);
        $query->execute();

        return $query->fetch();
    }
}

?>

By initializing the $pdo variable globally and ensuring it is used within the class methods, you will establish a proper database connection and resolve the "Call to a member function prepare() on null" error.