PHP Function for factorials but using addition instead of multiplication?

I know that the mathematical function for `5 * 4 * 3 * 2 * 1` is called factorial (and there is a PHP function for it: `gmp_fact(n)`). But what is it called when you are wanting to add instead of multiply? Like this: `5 + 4 + 3 + 2 + 1`? And is there a PHP function for this? Or do I need to roll my own? I assume there is a function for something like this but since I don't know what the mathematical name is for this kind of expression I can't seem to find the name of the function for it...