PHP: display leading zero

Hello everybody,
I'd like to list the numbers 01-31 to a drop down list(select box).
The code I'm using:

//////////////////////// snip
<SELECT name="kuup2ev">

<OPTION value="<?$kuup2ev = 01; echo $kuup2ev;?>" selected>
<?echo $kuup2ev;?>
</OPTION>
<? $date_select = $kuup2ev;  while($date_select < 32) { ?>
<OPTION value="<?echo $date_select; ?>"><?echo $date_select;?></OPTION>
<? $date_select++; } ?>

</SELECT>
//////////////////////// snip

Now, instead of:
01
02
03
04
...

I get:
1
2
3
4
...

The question: How can I make PHP display the leading zero in case the number is
only one digit?

Thanks,
Lauri

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