如何将javaScript值传递给PHP

Question

如何将javaScript值传递给PHP，我的代码是

 <script>
    function a{
    var b = a;
    }
   </script>

<button onclick="a(2)">Values</button>

<?php
    $select = mysql_query("select * from tabl1 where id='values'"); // values comes here
    echo $select;
    ?>