Understanding the ‘self’ keyword in PHP

In PHP, you use the self keyword to access static properties and methods.

The problem is that you can replace $this->method() withself::method() anywhere, regardless if method() is declared static or not. So which one should you use?

Consider this code:

class ParentClass {
 function test() {
  self::who(); // will output 'parent'
  $this->who(); // will output 'child'
 }

 function who() {
  echo 'parent';
 }
}

class ChildClass extends ParentClass {
 function who() {
  echo 'child';
 }
}

$obj = new ChildClass();
$obj->test();

In this example, self::who() will always output ‘parent’, while $this->who() will depend on what class the object has.

Now we can see that self refers to the class in which it is called, while$this refers to the class of the current object.

So, you should use self only when $this is not available, or when you don’t want to allow descendant classes to overwrite the current method.