If Load() was not static (and actually, even if it is), you could do a:
$class = $objCountry->AddressTableName;
$instance = new $class();
$objSelectedCountry = $instance->Load($intID);
That said, this looks like a rather crude form of polymorphism. There may be better ways of doing it, but we can only tell you if you give us the bigger picture of what you are trying to solve.