To allow protected and private properties to be accessed, you should use
ReflectionProperty::setAccessible(bool $accessible):

<?php
/** Class Foo with protected and private members */
class Foo {
    protected $bar = 'barrr!';
    private $baz = 'bazzz!';
}

$reflFoo = new ReflectionClass('Foo');
$reflBar = $reflFoo->getProperty('bar');
$reflBaz = $reflFoo->getProperty('baz');

// Set private and protected members accessible for getValue/setValue
$reflBar->setAccessible(true);
$reflBaz->setAccessible(true);

$foo = new Foo();
echo $reflBar->getValue($foo); // will output "barrr!"
echo $reflBaz->getValue($foo); // will output "bazzz!"

// You can also setValue
$reflBar->setValue($foo, "new value");
echo $reflBar->getValue($foo); // will output "new value"
?>