从javascript获取'var'到$ php

How can I get val value from javascript and add it in php?

Note: When I change selectbox -> alert showing for all values, it should show no thing when d == 1 but it show it for all.

<?php
    if ($rows['plus'] < $Stime OR $rows['plus'] == 0) {
        $saveal = mysql_query("UPDATE `".TB_PREFIX."users` SET plus = 0 WHERE id = \"".$userid."\"");
        echo "<b>غير مفعّل</b> ";
        $activ = array('يوم','يومان','3 أيام','4 أيام','5 أيام','6 أيام','7 أيام');
?> 
    <script language="javascript">
        function updateplus(){
            var d = document.getElementById("select_day").selectedindex;
            if (d == 0){
                var vla = 1;
            }else if (d == 1) {
                var val =2;
            }else{
                alert("please select value");
            }

        }
    </script>
    <select name="select_day" id="select_day" onchange="updateplus()">
  <?php
    foreach($activ as $key => $value):
        echo '<option value="'.$key.'">'.$value.'</option>'; //close your tags!!
    endforeach;
    //$saveal2 = mysql_query("UPDATE `".TB_PREFIX."users` SET plus = \"".$s."\" WHERE id = \"".$userid."\"");
    ?>
  </select>