PHPUnit check method returns type

I have a trouble to test what type of content returns the method. On success it returns string or int, at fail returns array. So code like this

public function createAddressBook($bookName)
{
    if (true)
    {
        return 'OK';
    }
    return ['error' => 'FAIL'];
}

The real code of the method is API request for new book ID that is int or string and i don't know what value will returns. My test code is like so:

public function setUp()
{
    $this->stub = $this->createMock(\My\Class::class);
}

public function tearDown()
{
    $this->stub = null;
}

public function testCreateAddressBook()
{
   $this->stub->method('createAddressBook')->with('TestBookName');
   $result = $this->stub->createAddressBook('TestBookName');
   //what to do now?
}

My question is how to test method for returning value type?