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

The problem is that you can replace $this->method() with self::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.