<?php

class A {
protected static $var = [];

public static function append($str) {
	$var = static::$var;
	static::$var = &$var;
	static::$var[] = $str;
}
public static function print() {
	print_r(static::$var);
}
}

class B extends A {
	
}

class C extends A {
	
}

class D extends A {
protected static $var = [];
}

B::append('B');
C::append('C');
D::append('D');

B::print();
C::print();
D::print();