function overloading

Heya, I would like to know If it is somehow possible to overload existing functions by extensions. And if it is possible, are there already some extension doing it? I am not talking about the magic __call function. I am talking about something like: let's assume GMP has overloaded the function "abs" and therefore one can write: $n = gmp_init(-12345678901234567890); abs($n+ 2); // instead of using gmp_abs I know that GMP has overloaded operators so it doesn't seem so far that overloading functions is possible as well. I hope someone can help me. Cheers, Robert