<?php
$list = [-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 10, 11, 14, 15, 17, 18, 19, 20];
for($i = 0;$i<count($list);$i++){
echo current($list)+1 . '<br>';
if(next($list)==current($list)+1){
echo $list[$i];
} else {
echo 'no'.PHP_EOL;
}
}
print_r($list);