how to assign js variable to a 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

<script type="text/javascript">
// boolean outputs "" if false, "1" if true
var bool = "<?php echo $bool ?>"; 

// numeric value, both with and without quotes
var num = <?php echo $num ?>; // 7
var str_num = "<?php echo $num ?>"; // "7" (a string)

var str = "<?php echo $str ?>"; // "A string here"
</script>