<?php
if ($_SERVER['REQUEST_METHOD'] != 'POST'): // Check if form has been submitted
?>
<!-- Show the form when user first visits site -->
<label> Enter the following info to continue:</label>
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
    <input type="text" name="roll_no" id="roll_no" /><br/>
    <input type="text" name="dob" id="dob" /><br/>
    <input type="submit"/>
</form>
<?php else: // Form has been submitted ?>

<!-- Do your stuff here
(Like use PHP to test validity of submitted data
and undertake various actions based on the data) -->

<h1> Hello! </h1>
Your Roll No is <?php echo $_POST['roll_no']; ?> <br/>
Your DOB is <?php echo $_POST['dob']; ?>

<?php endif; ?>