Static attributes (or methods) can be considered class definition attributes (or methods). They are not accessed from a Class instance but directly from the definition of the class itself, as such they can be accessed directly using the Class_Name::$attribute_name syntax or within the class by the Self::$attribute_name syntax.

The following are a couple of tests of this.

We create a simple class with an Static Attribute

class Static_Test {
	public static $counter = 0;
	function show_counter() {
		echo self::$counter;
	}
	function increase_counter() {
		self::$counter++;
	}
}

Accessing the static attribute from the class definition

echo Static_Test::$counter;

Results: 0

Accessing the static attribute within a method from an instance of the Class

$obj1 = new Static_Test;
$obj1->show_counter();

Results: 0

Modifying the attribute value

Static_Test::$counter++;
$obj1->show_counter(); //1

Results: 1

// $obj1->counter++; //Does Nothing.

Results: nothing. Static Attributes can not be accessed directly, even when declared as public.

$obj1->increase_counter();
$obj1->show_counter(); //2

Results: 2. They can be accessed within methods if the syntax used in the method is appropiate.

Accessing the static attribute within a method from another instance of the Class

$obj2 = new Static_Test;
$obj2->increase_counter();

$obj1->show_counter(); //3

Results: 3. Changes done to a static attribute on an instance are reflected across all instances of the class.

A graphical representation of what we’ve done

Accessing Static Attributes from a Child Class

class Static_Child_Test extends Static_Test {
	function increase_parent_counter() {
		parent::$counter++ ;
	}
}
$obj3 = new Static_Child_Test;
$obj3->increase_parent_counter();

$obj2->show_counter(); //4

Results: 4.

Accessing Static Methods

The same holds true for static methods. An example:

class Static_Test2 {
	public static function static_method() {
		echo 'This is an static Function. Called directly from the Class Definition and not from an Instance of the Class.';
	}
}

Static_Test2::static_method();