I have an array ($number_list) that has a dynamically generated list of values. There will be at least 1 value in the array and no more than 4.
Currentl
Try array_pop() to get the last element, then array_push to modify and push the element back:
http://php.net/manual/en/function.array-pop.php
Something like
$last_element = array_pop($number_list);
array_push($number_list, 'and '.$last_element);
Then you can do your implode:
$comma_list = implode(', ', $number_list);