How do I access the data of a field using PHP?
Language negotiation is handled for you by field_get_items, and the delta is above the field column in the array hierarchy (it goes $field[LANGUAGE][DELTA][COLUMN_NAME]). 

So using und isn't necessary, and you need to change how you're accessing each field item.

Code similar to this should work:

$field_lab = field_get_items('node', $node, 'field_lab_s_requested');
foreach ($field_lab as $field_item) {
  switch ($field_item['value']) {
    case "127":

      break;
    // ...
  }
}