store javascript variable into php variable 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 php variable in javascript file

<?php
    $color = "Red";
?>
<script type="text/javascript">var color = "<?= $color ?>";</script>
<script type="text/javascript" src="file.js"></script>