使用唯一编号打印HTML页面

I am having a php page in which i have captured different values from the previous page, There is also a print button, Which prints all these fields including the unique number. When user clicks on the print button record will be inserted in database and print window displays on the screen. But the problem is there is a unique number on html page, For example if two persons are login at the same time, The will get same unique number, and both will be able to print the same page having same unique number.

How i can control this issue, I also tried to redirect the user to the previous page but its not working.

Here is my php page, Which i am calling using ajax

<?php
$conn = mysql_connect('localhost','','') or die ("");
mysql_select_db("") or die ("Database Problem");
$query = "select * from print where a = $a;
$result = mysql_query($query);
if(mysql_num_rows($result) == 0)
{
    $query = "INSERT INTO print () VALUES () 
    $result = mysql_query($query) or die();
    if(mysql_affected_rows()<>1)
    {
        header ("Location:mainpage.php?uniquenumber=xy");
    }
}

you can use unisid http://php.net/manual/en/function.uniqid.php To generate an unique id

What is this uniquenumber needed for in the client side? I mean having it accessible and editable for the users is kinda dangerous if it's an important value.

If you just need this number to tell apart the different entries in the print table, why not just use and auto_increment index for this table?

Another solution would be session variables. When a user succesfully logs in generate this unique ID based on multiple variables (username, time, browser), that will ensure there won't be repeated values for this ID. Then store the variable like this:

session_start();
$_SESSION['ID']=$unique_ID;

You can then read it in any other PHP script like this:

session_start();
$unique_ID=$_SESSION['ID'];

You can validate the unique number before inserting to database ( Before Printing also ). If the unique no is exist, throw a error message "unique no is already used " and give another unique number ( By loading the HTML Page again or doing something else ).