Well technically Math operators are also functions as they have to be read by the compiler and then processed... I agree though that if you are doing a couple hundred thousand operations your code would be a bit quicker... but our codes do exactly the same thing. Personaly i would use your code...but it's basically the difference between
If(substr($n,-1) == 0)
Versus
If ( !($n % 10))
With no practical difference.