PHPUnit how to test protected methods?

Question

PHPUnit

Good day!
I can not understand 1 point in testing - protected and private methods.
About their testing they write something like this

Generally speaking, testing private and protected methods is a bad idea. They are not part of the public interface and are usually implemented through the public methods of the class. So when we test public methods, they implicitly test private and protected methods.

That is, either you have a bad implementation of the class, or test together with the public one.
But, if I have a public method, it uses a couple of private methods that form, for example, various data arrays. And it makes no sense to shine them outside. To test both methods, I need to generate a huge test just for the successful variant. And so I could test these methods separately and be sure of them. And in a public method, it's easy to mock. This greatly reduces the complexity of writing and understanding the test.
In the standard implementation, I did not find an option for such testing. Is on stackoverflow

protected static function getMethod($name) {
  $class = new ReflectionClass('MyClass');
  $method = $class->getMethod($name);
  $method->setAccessible(true);
  return $method;
}

public function testFoo() {
  $foo = self::getMethod('foo');
  $obj = new MyClass();
  $foo->invokeArgs($obj, array(...));
  ...
}

In my opinion, this is a crutch
. Can you explain why this is not implemented in the standard functionality. Or how can I write classes and tests for such options?