Question

I currently have a database wrapper that uses a Singleton, like so:

class Database {
    private static $db;

    public static function getInstance() {
        if(!self::$db) {
            self::$db = new PDO();
        }
        return self::$db;
    }
}

I also have a User class that has a few methods I want to call statically, but they require a database connection, which I’m doing like so:

class User {
    private static $db;

    public function __construct() {
        self::$db = Database::getInstance();
    }

    public static function someMethod() {
        self::$db->someQuery();
    }
}

User::someMethod();

My question is, how would I accomplish the same thing using Dependency Injection instead of Singleton, without making multiple DB connections?

Answer 1

You would simply pass the DB connection to the constructor like this:

class User {
    private $db;

    public function __construct($db) {
        if ($db instanceof Database) {
            $this->db = $db;
        } else {
            throw new Exception('Give me a Database object');
        }
    }

    public function someMethod() {
        $this->$db->someQuery();
    }
}