Best Way to Unit Test a Website With Multiple User Types with PHPUnit

class User
{
    public function commonFunctionality()
    {
        return 'Something';
    }

    public function modifiedFunctionality()
    {
        return 'One Thing';
    }
}

class SpecialUser extends User
{
    public function specialFunctionality()
    {
        return 'Nothing';
    }

    public function modifiedFunctionality()
    {
        return 'Another Thing';
    }
}

class Test_User extends PHPUnit_Framework_TestCase
{
    public function create()
    {
        return new User();
    }

    public function testCommonFunctionality()
    {
        $user = $this->create();
        $this->assertEquals('Something', $user->commonFunctionality);
    }

    public function testModifiedFunctionality()
    {
        $user = $this->create();
        $this->assertEquals('One Thing', $user->commonFunctionality);
    }
}

class Test_SpecialUser extends Test_User
{
    public function create() {
        return new SpecialUser();
    }

    public function testSpecialFunctionality()
    {
        $user = $this->create();
        $this->assertEquals('Nothing', $user->commonFunctionality);
    }

    public function testModifiedFunctionality()
    {
        $user = $this->create();
        $this->assertEquals('Another Thing', $user->commonFunctionality);
    }
}