<? foreach array("M", "L", "XL") as $size ?>
Купить
<a href="/add2basket.php?size=<?= $size ?>"><?= $size ?></a>
<? endfor ?>

<!-- или -->

<form action="/add2basket.php" method="post">
  <? foreach array("M", "L", "XL") as $size ?>
  <input id="size-<?= $size ?>" type="radio" name="size" value="<?= $size ?>" />
  <label for="size-<?= $size ?>"><?= $size ?></label>

  <input type="submit" value="Купить сичяс" />
</form>