I did the same and I though it will work better as "return array_sum(range(0,$n));" because we will not use a lot of memory for huge parameter $n in this case. Yes, it will not use memory, but it will use processor time for calculation.

Yes, it's a good solution in one line, but it does not use memory effective. We have a memory limit exception if parameter $n is huge number (for example, 1 Gb of memory is not enough if $n = 17 millions).