Question

From the 2nd element in $array, increment the key by 100, suppose the keys are all numeric.

Answer 1

$new_array = array();
$count = 0;
foreach ($original_array as $key => $value)
{
  if ($count > 0)
    $new_array[$key + 100] = $value;
  else
    $new_array[$key] = $value;
  $count++;
}

Now $new_array contains your “shifted” $original_array, starting from element #2.