- var_export: documentation ( source)
- explode: documentation ( source)
<?php
$str = '1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,28,29,30,31,32,33,34,35,36,37,38,39,41,45,47,71,73,75,76,77,78,80,81,83,87,88,89,90';
$list = explode(',', $str);
$list_new = array(91) + $list;
var_export($list_new);