How to read purticular variable without know variable name in PHP?

Question

Why I can't read static variables of a child class, even so the variable is protected or public?

Here's what I tried:

class A {
 protected static $foo = "bar";
}

class B {
 public function readFoo() {
 return self::$foo;
 }
}

When now calling B::readFoo, I get the following error:

var_dump((new B())->readFoo());

PHP Fatal error: Access to undeclared static property: B::$foo in php shell code on line 1
PHP Stack trace:
PHP 1. {main}() php shell code:0
PHP 2. B->readFoo() php shell code:1

Fatal error: Access to undeclared static property: B::$foo in php shell
code on line 1

Call Stack:
 86.2643 231224 1. {main}() php shell code:0
 86.2666 231352 2. B->readFoo() php shell code:1