Question

I have

class Check
{
    public function __construct()
    {
        $this->db = new Database();
    }

    public function query()
    {
        $login = Session::get("login");
        $sth = $this->db->prepare('SELECT admin FROM accounts WHERE login=:login');
        $sth->execute(array(':login' => $login));
        $result = $sth->fetch(PDO::FETCH_NUM);
        return $result[0];
    }

    public static function admin()
    {
        echo self::query();
    }
}

I have Database class in another place with PDO connection.

class Database extends PDO
{
    public function __construct()
    {
        parent::__construct('mysql:host=localhost;dbname=name','root','pass');
        $this->query('SET NAMES utf8');
    }
}

So after Check::admin() code I get error:

Undefined property: View::$db

Why?

Answer 1

You are using a static method, that wants to use a instance variable.

Your admin method calls the query method, and the query method is using the db instance variable. As your class is not instantiated, the db variable does not exists.

My suggestion would be to make the admin method non static and use your code like this:

$mycheck = new Check();
$mycheck->admin();

or, if you are on php 5.4 and want to stick with a oneliner:

(new Check())->admin();

update

note: Do not create the db class in the constructor, but inject it:

public function __construct(Database $db)
{
   $this->db = $db;
}