私人与私人静态

I always use private $MyVar = false; when declaring private variables which only that specific class can use. But recently i saw some examples where people use private static $MyVar = false;.

I don't get it, what's the difference? Both of them can only be used inside that class, so whats the point in adding static?

Compare:

1. A private instance variable named x:

   class Foo {
       private $x = 0;
       public function Foo() {
           echo ++$this->x . ',';
       }
   }
   
   new Foo();
   new Foo();
   

   The output is 1,1,.

2. A private static variable named x:

   class Bar {
       private static $x = 0;
       public function Bar() {
           echo ++self::$x . ',';
       }
   }
   
   new Bar();
   new Bar();
   

   The output is 1,2,.