When to use self over $this?

Question

In PHP 5, what is the difference between using self and $this?

When is each appropriate?

Answer 1

The keyword self does NOT refer merely to the 'current class', at least not in a way that restricts you to static members. Within the context of a non-static member, self also provides a way of bypassing the vtable (see wiki on vtable) for the current object. Just as you can use parent::methodName() to call the parents version of a function, so you can call self::methodName() to call the current classes implementation of a method.

class Person {
    private $name;

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

    public function getName() {
        return $this->name;
    }

    public function getTitle() {
        return $this->getName()." the person";
    }

    public function sayHello() {
        echo "Hello, I'm ".$this->getTitle()."
";
    }

    public function sayGoodbye() {
        echo "Goodbye from ".self::getTitle()."
";
    }
}

class Geek extends Person {
    public function __construct($name) {
        parent::__construct($name);
    }

    public function getTitle() {
        return $this->getName()." the geek";
    }
}

$geekObj = new Geek("Ludwig");
$geekObj->sayHello();
$geekObj->sayGoodbye();

This will output:

Hello, I'm Ludwig the geek
Goodbye from Ludwig the person

sayHello() uses the $this pointer, so the vtable is invoked to call Geek::getTitle(). sayGoodbye() uses self::getTitle(), so the vtable is not used, and Person::getTitle() is called. In both cases, we are dealing with the method of an instantiated object, and have access to the $this pointer within the called functions.