PHP 类的静态变量索引

直接上代码

<?php
class example{
    public static $pa;
    public $pb;
    public function __construct(){
        $this->pb = ++self::$pa;
    }
}

$a = new example;
$b = new example;

echo $a->pb;
echo '<hr/>';
echo $b->pb;
?>

 本来想结果应该是

1
-----------------------------------------------------
1

可是我错了，正确的结果是

1
----------------------------------------------------------------------------------
2

基础没学好，赶快补补