unique identifier

[poLka]

ok, i am looking for your (yes you!) opinion on this method of determining a unique visitor. i am just looking for an alternative to session variables and/or cookies. what i need is to generate an order_id for each user that is unique (never will one be the same, ever!), and then all their file uploads will be associated with this order_id.

so what i did was create an order info table that had an auto_increment order_id field and another field for the date (to seconds)... and this is my script to generate a unique order_id.

//get time
$order_time = date("n:d:Y G:i:s");

//set order_id
$sql = "INSERT INTO order_info (order_time, checked) VALUES ('$order_time', 'yes')";
mysql_query($sql,$conn) or die("could not create order id ".mysql_error());

//get order_id
$sql = "SELECT order_id FROM order_info WHERE order_time = '$order_time'";
$r = mysql_fetch_row(mysql_query($sql,$conn)) or die("could not select order id ".mysql_error());
$order_id = $r[0]; 

the only problem i can think of is that if two users sign on to the system at the exact same second then this system has failed (but that is easy to check for). what are your opinions to this system and do you have any suggestions. all comments will be appreciated.

[sylow]

I think you should start reading mysql documentation first before proceeding further. Because mysql does have last insert id function which returns the last insert id (as name says) for the last query. And it is safe method because no 2 person gets the same value.
http://www.php.net/manual/en/functio...-insert-id.php

[poLka]

thanks for the heads up bro, i'll definetely look into that.

[siteguru]

Here's another way to generate a unique id ...

/* Create a Unique 32-character ID */
$uniqueid = md5 (uniqid (rand)); 

... not saying it's any better - just another way.

[poLka]

thanks siteguru, with that statment alone, there is no assurance that a previously generated id will resurface itself, correct? i apologize for my ignorance beforehand, i am not familiar with the md functions.

[siteguru]

uniqid() returns a prefixed unique identifier based on the current time in microseconds. The prefix can be useful for instance if you generate identifiers simultaneously on several hosts that might happen to generate the identifier at the same microsecond. Prefix can be up to 114 characters long.

If called without the optional min, max arguments rand() returns a pseudo-random value between 0 and RAND_MAX. If you want a random number between 5 and 15 (inclusive), for example, use rand (5, 15).
In older versions of PHP, you had to seed the random number generator before use with srand(). Since 4.2.0 this is no longer necessary.

md5() calculates the MD5 hash of str using the RSA Data Security, Inc. MD5 Message-Digest Algorithm, and returns that hash. The hash is a 32-character hexadecimal number.

That's the official explanation.
Basically it means it is virtually impossible to get two identical results.

[poLka]

thanks bro! much appreciated.

[poLka]

what would be your suggestions to keeping that unique identifier throughout a users "session" , am thinking about letting them navigate throughout the site via forms to allow for POST data, but what if they hit the "Back" button on the browser?

[siteguru]

Store the unique id as a session variable ...

// (Re-)Start the session handler (this must be on all pages using sessions)
session_start();

// Set the session variable (e.g. after login)
$_SESSION['unique'] = "the unique id value";

// To check for it
if ( empty ($_SESSION['unique']) !== false ) {
  // The session variable is empty
}