oop - Where's the difference between self and $this-> in a PHP class or PHP method?

Question

Where's the difference between self and $this-> in a PHP class or PHP method?

Example:

I've seen this code recently.

public static function getInstance() {

    if (!self::$instance) {
        self::$instance = new PDO("mysql:host='localhost';dbname='animals'", 'username', 'password');;
        self::$instance-> setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
    return self::$instance;
}

But I remember that $this-> refers to the current instance (object) of a class (might also be wrong). However, what's the difference?

Answer 1

$this refers to the instance of the class, that is correct. However, there is also something called static state, which is the same for all instances of that class. self:: is the accessor for those attributes and functions.

Also, you cannot normally access an instance member from a static method. Meaning, you cannot do

static function something($x) {
  $this->that = $x;
}

because the static method would not know which instance you are referring to.