Callling callbacks

What is the best way to have a function accept an arbitrary callback (be  
it a string, array or closure in case of PHP 5.3) that is compatible with  
PHP 5.2 and 5.3?

The code for array_walk and call_user_func is quite different in the two  
branches. Do I have to include both variants or is there some abstraction  
that works in both branches?

Thanks

-- 
Gustavo Lopes