You cant access PHP session variables/values in JS, one is server side (PHP), the other client side (JS).
What you can do is pass or return the SESSION value to your JS, by say, an AJAX call. In your JS, make a call to a PHP script which simply outputs for return to your JS the SESSION variable's value, then use your JS to handle this returned information.
Alternatively store the value in a COOKIE, which can be accessed by either framework..though this may not be the best approach in your situation.
OR you can generate some JS in your PHP which returns/sets the variable, i.e.:
<? php
echo "<script type='text/javascript'>
alert('".json_encode($_SESSION['msg'])."');
</script>";
?>