how to store value of javascript variable in php variable code example

Example 1: how to assign value of a js variable to a php variable

<script> document.cookie = "myJavascriptVar = " + myJavascriptVar </script>
<?php
     $myPhpVar= $_COOKIE['myJavascriptVar'];
?>

Example 2: javascript variable store in php

<script>
var a="Hello";
</script>