MyObjectUnitTest.php

<?php

class MyObjectUnitTest extends UnitTestBase {

  /**
   * Mock object for one of the objects dependencies.
   *
   * @var \Project\Dependency|\Prophecy\Prophecy\ObjectProphecy
   */
  protected $dependency;

  /**
   * {@inheritdoc}
   */
  public function setUp() {
    $this->dependency = $this->prophesize(Dependency::class);
  }

  /**
   * Create a new object, optionally with some methods mocked.
   *
   * @param string[] $mock_methods
   *   List of method names to mock.
   *
   * @return \Project\MyObject|\PHPUnit_Framework_MockObject_MockObject
   *   Mock object wrapping the real object.
   */
  protected function createMyObject($mock_methods = []) {
    $report_generator = $this->getMockBuilder(Object::class)
      ->setConstructorArgs([
        $this->dependency->reveal(),
      ])
      ->setMethods($mock_methods)
      ->getMock();

    return $report_generator;
  }

  public function testProtectedMethod() {
    // Create the object under test, but we're going to mock one of it's internal methods
    // so that our test *only* calls the method under test, and any other methods that
    // get called are mocked (in this case, there's only one).
    $object = $this->createMyObject(['internalMethod']);
    $object->method('internalMethod')
      ->willReturn(['value' => 'test']);
    
    $result = $this->invokeMethod($object, 'protectedMethod', ['argument1']);
    $this->assertEqual("expected value", $result);
  }

}

UnitTestBase.php

<?php

abstract class UnitTestBase extends \PHPUnit_Framework_TestCase {

  /**
   * Call protected/private method of a class.
   *
   * @param object &$object
   *   Instantiated object that we will run method on.
   * @param string $methodName
   *   Method name to call
   * @param array $parameters
   *   Array of parameters to pass into method.
   *
   * @return mixed
   *   Method return.
   */
  public function invokeMethod(&$object, $methodName, array $parameters = array())
  {
    $reflection = new \ReflectionClass(get_class($object));
    $method = $reflection->getMethod($methodName);
    $method->setAccessible(true);

    return $method->invokeArgs($object, $parameters);
  }

}