If no-one comes up with a sensible answer, I'd try:
$file = "/nicole/horoscopes/" . $_POST['ddate'] . ".txt";
And in your form just use
<option>Aries mar 21 - apr 19</option>
instead of
<option name="ari">Aries mar 21 - apr 19</option>
and you don't need the size in the following
<select name="ddate" size="1">