Description:
------------
The goal is to return all declared subclasses of given class_name/interface


Reproduce code:
---------------
interface A {}
class A1 implements A {}
class A2 implements A {}
class B extends A1 {}
class C {}

print_r(get_declared_classes('A'));
print_r(get_declared_classes('A1'));

Expected result:
----------------
// print_r(get_declared_classes('A'));
Array
(
    [0] => A1
    [1] => A2
    [2] => B
)

// print_r(get_declared_classes('A1'));
Array
(
    [2] => B
)