how to assign javascript variable to PHP variable [duplicate]

I am generating dynamic rows with add more function. In JavaScript I am fetching MySQL data and I generate a dropdown. For executing the query I need a php variable value, but I have a JavaScript value. So, to execute the query I need php value. I need help to assign JavaScript variable to the php variable.

I have tried this way:

<script>
var j = 'this is JavaScript value';

<?php $ABC = "<script>document.write(j)</script>"?>