How to call function in another class

Question

how to call to a function in another class. I can't explain, but i hope you will understand my problem.

Notice: Undefined variable: db in class general.

http://pastebin.com/C1aY1US5

Answer 1

Inject it:

class general{
    public function __construct(db $db){
        $this->db = $db;
    }
    public function userId($nickname) {
        $result = $this->db->query('SELECT `id` FROM `users` WHERE `username` = $nickname'); //there i want to call to db class

        return($this->db->fetch($result));
    }
}

And later:

$db = new db();
$general = new general($db);