<script>
var my_selectbox=document.getElementById('my_selectbox');
my_selectbox.onchange=function() {
if (this.options.selectedIndex==0) {
    document. cookie =
  'my_cookie=testcookie1; expires=Thu, 6 Aug 2009 20:47:11 UTC; path=/';
 }
 else if (this.options.selectedIndex==1) {
    document. cookie =
  'my_cookie=testcookie2; expires=Thu, 6 Aug 2009 20:47:11 UTC; path=/';
 }
 else if (this.options.selectedIndex==2) {
    document. cookie =
  'my_cookie=testcookie3; expires=Thu, 6 Aug 2009 20:47:11 UTC; path=/';
 }
}
</script>
