<?php
$s = "1,2,3,8,2";
$ints = array_map('intval', explode(',', $s ));
var_dump( $ints );

array(5) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
  [3]=>
  int(8)
  [4]=>
  int(2)
}