<?php
class A {
 protected static $def = '123456';

 public static function test() {
  echo get_class(new static);
 }

 public static function test2() {
  echo static::$def;
 }
}

class B extends A {
 protected static $def = '456789';
}

class C extends A {
 protected static $def = 'abcdef';
}

echo B::test();
echo '<br>';
echo C::test();
echo '<br>';
echo B::test2();
echo '<br>';
echo C::test2();
echo '<br>';
echo A::test();
echo '<br>';
echo A::test2();
echo '<br>';

// 输出结果
B
C
456789
abcdef
A
123456