Math average with php

array_sum($values) / count($values)

If what you mean by average is the 'mean' and you don't want to store all numbers then store their count:

$last_average = 100;
$total_numbers = 10;
$new_number = 54;

$new_average = (($last_average * $total_numbers) + $new_number) / ($total_numbers + 1);