is_a() against instanceof

Hi, PHP5 uses the 'instanceof' keyword instead of the deprecated is_a() function. But there's a big difference between the two: - is_a($cls,"ClassName") *doesn't require* from ClassName to be declared, and return false when ClassName is not declared. - ($cls instanceof ClassName) *requires* from ClassName to be declared, and generates a fatal error when ClassName is not declared. Since is_a() is deprecated.. is there a way to use instanceof exactly like is_a() function (so it will not make an error when the class is not declared)? -thanks, Eli