Pass dynamic javascript variable to PHP

Question

I wanna pass a JavaScript variable in to PHP, that's my code:

$('.info-button').on('click', function() {
         var flag = $(this).data('flag');

            if (flag) {
                var index3 = $(this).parent().parent().parent().find('img').attr('src');
                console.log(index3);
              $('.inner').css("display", "block");
            } else {
                    $('.inner').css("display", "none");
            }

            $(this).data('flag', !flag);
});

How to get index3, which is dynamic variable from PHP?

I tried with:

<?php
   echo "<script>document.writeln(index3);</script>";
?>

But Im getting index3 is not defined