When to use Self vs $this in PHP?

I see as a very frequent doubt:

When should we use self::, or $this in PHP. Which shape is more suitable for use and what is the difference of the 2 situations?

Author: William Aparecido Brandino, 2013-12-18
Source

5 answers

In simplified Form, $this refers to the current object (instance), and self refers to the class. Therefore, as a general rule, one uses $this to access members (attributes, methods) of the instance and self to access static members.

Inheritance

When using inheritance, however, there is a difference between using self and $this when calling an instance method:

  • self::metodo() calls the metodo() of the current class;
  • $this->metodo() calls the metodo() of the class used to instantiate the object being executed (which can be a subclass of the class where the call is made). You can find out what class this is by using get_class($this).

Example: 

<?php
class Animal {
  public function teste() {
    echo "\$this é instância de " . get_class($this) . "\n";

    // chama Animal::fala(), independentemente do
    // tipo da instância
    echo "self::fala(): ";
    self::fala();

    // chama fala() na classe usada pra instanciar
    // este objeto
    echo "\$this->fala(): ";
    $this->fala();
  }
  public function fala() {
    echo "Oi\n";
  }
}

class Gato extends Animal {
  public function fala() {
    echo "Miau\n";
  }
}

// Nesse caso, self != get_class($this)
// - self == Animal
// - get_class($this) == Gato
$gato = new Gato();
$gato->teste();

echo "\n";

// Nesse caso, self == get_class($this) == Animal
$animal = new Animal();
$animal->teste();
?>

Result: 

$this é instância de Gato
self::fala(): Oi
$this->fala(): Miau

$this é instância de Animal
self::fala(): Oi
$this->fala(): Oi
 64
Author: rodrigorgs, 2013-12-18 11:41:13

The difference is that self is for when the class (or instance), is in a static context (be it a method or property), and obviously this is when it is not static.

 5
Author: Brayan, 2013-12-18 11:16:01

$this it is used within the class to access object properties / methods. self is used to access static members.

 3
Author: rray, 2013-12-18 11:53:06

$this points to the object and self points to the class itself.

Self can also be used when the class extends another and you want to access its implementation or its relative for example 

self::teste();

Or 

parent::teste();

But usually self will be used to access static data of the class.

 3
Author: Guerra, 2013-12-20 11:16:08

In general, we use $this to maintain encapsulation and avoid colliding information from one object into the other. If for some reason you need to share information with all instances of a class, that's where the static variables come in, let's look at an example:

<?php

class Notebook
{
    private static $quantidadeDisponivel = 5;

    public function getQuantidadeDisponivel()
    {
        return self::$quantidadeDisponivel;
    }

    public function comprar()
    {
        /**
         * Realiza o processo de compra do notebook e atualiza quantidade disponivel
         */
        self::$quantidadeDisponivel--;
    }
}

$carrinhoJoao = new Notebook;
$carrinhoJoao->comprar();
$notebooksRestantes =  $carrinhoJoao->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 4

$carrinhoPedro = new Notebook;
$carrinhoPedro->comprar();
$notebooksRestantes =  $carrinhoPedro->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 3

$carrinhoPaulo = new Notebook;
$carrinhoPaulo->comprar();
$notebooksRestantes =  $carrinhoPaulo->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 2

$carrinhoJoao->comprar();
$notebooksRestantes =  $carrinhoJoao->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 1

$carrinhoPedro->comprar();
$notebooksRestantes =  $carrinhoPedro->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 0

Without the use of variable statica, it would not be so simple to do this control, see: 

class Desktop
{
    private $quantidadeDisponivel = 5;

    public function getQuantidadeDisponivel()
    {
        return $this->quantidadeDisponivel;
    }

    public function comprar()
    {
        /**
         * Realiza o processo de compra do notebook e atualiza a quantidade disponível
         */
        $this->quantidadeDisponivel--;
    }
}



$carrinhoJoao = new Desktop;
$carrinhoJoao->comprar();
$notebooksRestantes =  $carrinhoJoao->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 4

$carrinhoPedro = new Desktop;
$carrinhoPedro->comprar();
$notebooksRestantes =  $carrinhoPedro->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 4

$carrinhoPaulo = new Desktop;
$carrinhoPaulo->comprar();
$notebooksRestantes =  $carrinhoPaulo->getQuantidadeDisponivel(); 
print $notebooksRestantes; // imprime 4

As we have seen, with the use of the static variable we had the result 4,3,2,1,0 and with the common variable we had 4,4,4, the computers would continue to be sold even if they were finished.

(this example is merely educational!)

 3
Author: Ed Cesar, 2017-01-27 23:47:33