how static variable works in PHP.

static_variable.php

<?php
class Base
{
  static public $name = 'BASE';
  function stamp($name) {
    static::$name = $name;
  }
}

class A extends Base
{
  static public $name = 'A';
}

class B extends Base
{
  static public $name = 'B';
}

$a = new A;
$b = new B;
assert('A' === $a::$name);
assert('B' === $b::$name);
$a->stamp('Z');
assert('Z' === $a::$name);
assert('B' === $b::$name);


class C extends Base {}
class D extends Base {}

$c = new C;
$d = new D;
assert('BASE' === $c::$name);
assert('BASE' === $d::$name);
$c->stamp('Z');
assert('Z' === $c::$name);
assert('Z' === $d::$name);