How to pass js variables to PHP code example

Example 1: js var to php

<script>
   var res = "success";
</script>
<?php
   echo "<script>document.writeln(res);</script>";
?>

Example 2: how to use javascript variable in php

<script>
var p1 = "success";
</script>

<?php
echo "<script>document.writeln(p1);</script>";
?>