静态 static

<?php
class A {
public static $instance;
public static function getInstance()
{
if(null === static::$instance){
static::$instance = new static();
}
return static::$instance;
}
}
class A1 extends A {}
class A2 extends A {}

/*****/
class B {
public static function getInstance()
{
static $instance;
if(null === $instance){
$instance = new static();
}
return $instance;
}
}
class B1 extends B{}
class B2 extends B{}

$a1 = A1::getInstance(); $b1 = B1::getInstance();
$a1_c = A1::getInstance(); $b1_c = B1::getInstance();
$a2 = A2::getInstance(); $b2 = B2::getInstance();
var_dump($a1, $a1_c, $a2, $b1, $b1_c, $b2);

var_dump($a1 === $a1_c); var_dump($b1 === $b1_c);
var_dump($a1 === $a2); var_dump($b1 === $b2);
请问四个结果是如何？why?