Exposing protected and private object methods using \Closure::bindTo(). Code below requires PHP 5.6+ due to argument unpacking (...$args). Could probably be written for PHP 5.4+ but didn't want to reinvent argument unpacking, else could also just use the Reflection implementation.

xray.php

<?php

class User {
  private $firstname;
  private $lastname;
  public function __construct($f, $l) {
    $this->firstname = $f;
    $this->lastname = $l;
  }

  private function buildFullName() {
    return sprintf('%s %s', $this->firstname, $this->lastname);
  }

  private function greeting($say) {
    return sprintf('%s, %s', $say, $this->buildFullName());
  }

  private function greetingToUser($say, User $user) {
    return sprintf('%s from %s', $this->greeting($say), $user->buildFullName());
  }
}

/**
 * Exposes result of a protected/private method call outside the
 * object's scope.
 *
 * @param object $object Object on which to call method
 * @param string $method Method to call
 * @param array $args Arguments to be unpacked and passed to Method
 * @return mixed Result of invoking Method on Object given Arguments
 */
function xray($object, $methodToExpose, array $args = []) {
  $fn = function () use ($methodToExpose, $args) {
    return $this->$methodToExpose(...$args);
  };
  $exposed = $fn->bindTo($object, $object);
  return $exposed();
};

// The traditional way
function xray_reflection($object, $methodToExpose, array $args = []) {
  $m = new \ReflectionMethod($object, $methodToExpose);
  $m->setAccessible(true);
  return $m->invoke($object, ...$args);
};

$john = new User('John', 'Kary');
$chris = new User('Chris', 'Escalante');

echo implode("\n", [
  xray($john, 'greeting', ['Hello']),
  xray_reflection($chris, 'greetingToUser', ['Hello', $john]),
]);

// Output:
// Hello, John Kary
// Hello, Chris Escalante from John Kary

Thanks for this!
I ran into the problem of the (new ReflectionMethod($object, 'method'))->invoke(...) method not respecting declare(strict_types=1) whereas this Closure method does 👍