Description:
------------
Hi!

Currently when we extends on abstract class we cannot test if all conditions are met before instantiate the class.

This is not usefull, can you improve ReflectionClass::isInstantiable or create new method for this feature?

Test script:
---------------
class classTest extends abstractTest {
  // Missing methodTest
}

abstract class abstractTest {
  abstract public function methodTest();
}

Expected result:
----------------
class classTest extends abstractTest {
  // Missing methodTest
}

abstract class abstractTest {
  abstract public function methodTest();
}

$reflectionClass = new ReflectionClass('calssTest');
if($reflectionClass->IsInstantiable()) {
  // OK
} else {
  // Cutom error
}

Actual result:
--------------
Fatal error: Class classTest contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (abstractTest::methodTest)