Php variable from selected

trying to do a simple change value code based on a <select> selection for instance

<html>
<head>
</head>
<body>

<input name=“letter” value="">

<select name=“select”>
<option>a</option>
<option>b</option>
</select>

</body>
</html>

if option “a” is selected, teh value for “letter” will be “A” , and if “b” will be “B”. this is for a form but using a external pearl file for the mailing so there is already values in the html , so using the $post variable probably wouldnt work in my case. I guess all i really need to know is how to make a variable out of the selects.