the reason it returned blank is that there were some fatal errors in that code and the
display_errors has been set to
false in the php.ini file of your website
i tried debuggin your codes try this now
<?php
ini_set('display_errors', true);
session_start();
// store session data
$_SESSION['unique id']=12;
?>
<html>
<body><?php
//retrieve session data
echo "This is your unique identity". $_SESSION['unique id'];
?></body>
</html>
and this was what it displayed in the web browser
This is your unique identity12the html codes,
<html>
<body>This is your unique identity12</body>
</html>