Are you looking for something like this?
PHP Code:
<html>
<script language=javascript>
function add() {
frm.s.add(new Option(frm.new_value.value, frm.new_value.value));
}
function remove() {
frm.s.remove(frm.s.selectedIndex);
}
</script>
<br><br>
<form name=frm>
<select name=s>
<option>Please select...</option>
</select>
<br><br>
<input type=button value=" Add " onclick="add();"> New Option Value <input type=text name=new_value><br>
<input type=button value=" Remove " onclick="remove();">
</form>
</html>