Access a non-static property from a static method

Question

class database{
    protected $db;

    protected function connect(){
        $this->db = new mysqli( /* DB info */ ); // Connecting to a database
    }
}

class example extends database{
    public function __construct(){
        $this->connect();
    }

    public static function doQuery(){
        $query = $this->db->query("theQuery");   // Not working.
        $query = self::$db->query("theQuery");   // Not working.
        $query = parent::$db->query("theQuery"); // Also not working.
    }
}

I want to do something like that but I cant find a way that works, The property has to static...

Answer 1

You may access by instantiating a new object ($self = new static;). The sample code:

class Database{

    protected $db;

    protected function connect(){
        $this->db = new mysqli( /* DB info */ ); // Connecting to a database
    }
}


class Example extends Database{

    public function __construct(){
        $this->connect();
    }

    public static function doQuery(){

        $self = new static; //OBJECT INSTANTIATION
        $query = $self->db->query("theQuery");   // working.

    }
}

This is the same as calling $self = new Example; but more programmatically, if the class name is ever changed it does not need updating.

Answer 2

You cannot access non-static properties from static methods. Non-static properties belong only to instantiated objects, where every instantiated object has a separate property value.

I will illustrate on an example, this code doesn't work:

class Example {
    public $a;

    public function __construct($a) {
        $this->a = $a;
    }
    public static function getA() {
        return $this->a;
    }
}

$first = new Example(3);
$second = new Example(4);

// is $value equal to 3 or 4?
$value = Example::getA();