How to create flash message in Wordpress in PHP with session?

Create flash message in Wordpress in PHP.

<?php 

if(isset($_POST['submit'])){

        session_start();

$info = '<div class="success">Application has been updated successfully.</div>';

$_SESSION['flashmessage'] = $info;

exit();

}

session_start();

if(!empty($_SESSION['flashmessage'])){ 

//echo 'jassi';

echo $_SESSION['flashmessage']; 

unset( $_SESSION['flashmessage'] );

/* session_unset();

session_destroy(); */

}

?>