in that case in order to passe the tests we shall do to int conversion, will it still be the best practice for this exercice? I used " function summation($n) : int "

The good thing about this very "vanilla" solution is that it requires less memory allocations (by not using array_map for example).

That's great. But to better protect against overflow with very big $n, i like to write it as "$n/2 * ($n+1)", this way $n will never multiply with $n+1 before the division.