PHP - Destroying Sessions properly on un-submitted form pages?

Question

If I have a php contact form with session_start() at the top, I know this creates a session. But what if the user doesn't fill out the form and instead navigates to a different page?

Do I still need to use session_destroy since I only want a session created when a user submits a php form via my contact page?

Thanks.

UPDATE: For a better idea on my form without posting lengthy code.

contact-form.html

<?php session_start(); ?>
<?php $fname = isset($_SESSION['fname'] ) ? $_SESSION['fname'] : NULL ; ?>
<form method="post" action="http://www.mysite.com/form-process.php">
  <input value="<?php echo $fname ?>" type="text" id="fname" name="fname" />
  <input type="submit" value="Submit Request" />
</form>

form-process.php

<?php
session_start(); 
$_SESSION['fname'] = $_POST['fname'];
$user = "John" ;
session_write_close(); 
if ($_SESSION['fname'] != $user) {
   header('Location: http://www.mysite.com/contact-form.html');
} 
else {
  $_SESSION = array();
  session_destroy();
  header('Location: http://www.mysite.com/thankyou.html');
}
?>

Answer 1

The overhead of creating a session is miniscule, there's no real reason you'd need to session_destroy() though you could put the session_start() in the block that detects post rather than at the top of the script if you only want to use the session when the user posts.