Question

I am new to OOP (PHP) and just met the design pattern – singleton.
I have found a DB class which uses mysqli (singleton class). I have added some custom methods to it (insert_id(), query(), fetch_result(), etc).

Then I created a new class called UserTools and I want to extend the database class to use the methods I’ve created previously (query(), fetch_result(), etc).
But I get this error:

Fatal error: Call to private Database::__construct() from invalid context in (…)
when I try to create instance of the new class (User Tools).

What should I do? Is it a right structure?

Answer 1

There are several way to achieve what you want.

One would be :

class UserTools {
    private $db;
    function __construct() {
        $this->db = Database::db_connect();
    }

    function login() { /* ... */}
}

Although it would be better to directly pass the database instance to the constructor like this :

class UserTools {
    private $db;
    function __construct($db) {
        $this->db = $db;
    }

    function login() { /* ... */}
}

// Usage 
$userTools = new UserTools(Database::db_connect());

If you’re really lazy you could just modify your database class and make the constructor public :

class Database {
    /* ... */
    public function __construct(){/* ... */}
    /* ... */
}

class UserTools extends Database {/* ... */}

But I really discourage you to use the latter one. It’s really bad code and it doesn’t make sense in a logical point of view. Your UserTools class use a database instance. It is not a database.