Passing javascript variable within a javascript function to PHP

I have the following javascript code. The variable dyna was originally retrieved from PHP. As the user clicks any of the buttons, the variable dyna increases/decreases by 1 with each click (depending on whether they hit the '-' button or the '+' button). My objective is to pass the javascript variable 'dyna' through a POST method to PHP.

Any advice on how this can be done?

 <script>

 var dyna = <?php echo $user_dyna; ?>

 $(function() {

 $(".numbers-row").append('<div class="inc button">+</div><div class="dec button">-</div>');

 $(".button").on("click", function() {

  var $button = $(this);

   if ($button.text() == "+") {
     dyna -= 1;
   } else {
       dyna += 1;
       } 

       $.ajax({
       url: 'stats.php',
       type: 'POST',
       data: {dyna: dyna}
       });

 });

});
 </script>