Changing an array of checkboxes to a single dropdown menu

Hi everyone, just getting into PHP and need to get pointed in the right direction.

I am trying to convert an array of checkboxes to a single dropdown menu. The checkboxes are being called in div .carttd6 Any help will be greatly appreciated!

Below is the snippet of code I am working on:

<?
$e=0;
$getRsss = mysql_query("SELECT optionals.* FROM optionals,optional_product where optional_product.prodid=".$rsp['id']." and optional_product.optid=optionals.id order by optional asc");
while ($rsss = @mysql_fetch_array($getRsss)) {
$e++;


if (in_array($rsss['id'],$extras_array)) $extra_total=$extra_total+($rsss['price']*$rss['qty']);


if ($e==1) {
?>
<div class="carttr2">
  <div class="carttd5"><!--tdcolspan4--><br clear="all" />
    <i>
    <?=$GLOBALS['extras']?>
    </i></div>
  <!--endtd5--> 
</div>
<!--endtr2-->
<? } ?>
<div class="carttr3">
  <div class="carttd6" style="vertical-align:middle;line-height:14px;">
    <input type="checkbox" name="extra_<?=$rsss['id']?>" id="extra_<?=$rsss['id']?>" value="<?=$rsss['id']?>" class="extras_cb cb_<?=$rss['id']?>" onclick="updateExtras(<?=$rss['id']?>);" <? if (in_array($rsss['id'],$extras_array)) echo "checked='true'"; ?>  />
    <?=$rsss['optional']?>
    [
    <?=setPrice($rsss['price']);?>
    ]</div>
  <!--endcarttd6-->
  <div class="carttd7" style="width:20px;"></div>
  <div class="carttd8" style="text-align:right;width:50px;"></div>
  <div class="carttd9" style="width:10px;"></div>
</div>
<? } 
}?>

Code (markup):

Attached is the full file, thanks in advance for any help provided.

Cheers