PHP - Uniqid(); Wtf?

I would like to secure my files from being downloaded using this code.

Code: [Select]
<?php
  $unique = md5( uniqid() );  // 32 hex characters
?>
but i dont know how to apply it to my files.

I have a form on page 1 that submits to page 2, then on to page 3, 4 and 5. On each page more data is collected, then page 5 puts it all into its respective place in the database.

I'm trying to prevent a duplicate entry from someone hitting the back button, and I've seen suggestions to do it with sessions and a uniqid.

I'm not versed in sessions, so my first question is, because I'm not submitting to the same page, where do I put the session? on page 2 or page one?
Right now, on page one, all I have is:

Code: [Select]
<?php session_start(); 

include("dbconnection.php"); 
?>

<!-- a bunch of  javascript form validation, html code and the form -->

<?php
  
 $unique_id = uniqid (rand (),true);
  $_SESSION['unique_id']=$unique_id;
  ?>

<form name="register1" class="registration_form" method="post" action="register2test.php" target="_self" onsubmit="return myForm()">
<input type="hidden" name="unique_id"  id="unique_id_form" value="<?php echo $unique_id; ?>"  >
<input type="submit" value="Submit"  class="buttontype"/>
</form>


on page two, I'm assuming, it's something similar to...

Code: [Select]
<?php
session_start();

if (isset($_POST["submit"]))
{
    if ($_POST["unique_id_form"] == $_SESSION["unique_id"])
    {
        $_SESSION["unique_id"] = '';
       /*set variables here ? */    }
    else
        echo 'error';
}
else
{
    $_SESSION["unique_id"] = uniqid (rand (),true);
?>