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copied!<p>First of all I would go for an auto increment too. Even if you choose to use a code as mentioned (month + random stuff) I would advise to also use a numeric (int/bigint) auto increment id. You should use this id 'below the surface' for your code to communicate with the database (referencing/looking up records).</p> <p>So for the 'month + random stuff' identifier I wouldn't advice to use anything with <code>uniqid()</code> for it is based on microseconds and not unique if two transactions would take place at the same microsecond. Also I would discourage the use of <code>md5()</code>, because this function would be very useful for hashing, but slows your code down a lot if you'd only use it for generating chars.</p> <p>Also your own code suggestion generates 4 random chars that could be 0-9 or a-f (hexadecimal) which gives you 16 possibilities. 4 chars of 16 possibilities gives you 16x16x16x16 = 65,536 unique strings. It would be highly possible that when rendering 10,000 of these strings you'd get duplicates.</p> <p>I would advice to generate a random string yourself from the chars 0-9, a-z and A-Z, which gives you 36 possibilities per char and with 4 chars gives you 36x36x36x36 = 1,679,616 unique strings. More chars gives you less probability for duplicates (collision). You could do such thing like this:</p> <pre><code>function randString($length = 4) { $randChars = 'abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'; $randString = ''; while (strlen($randString) < abs(intval($length))) { $randString .= substr($randChars, rand(0, strlen($randChars)-1), 1); } return $randString; } $id = date('m') . randString(4); </code></pre> <p>I would also leave out the dash ('<code>-</code>') between the month and the random chars because this saves you 1 char and you know the first two chars will always be the number of the month.</p> <p><strong>Edit: done some tests</strong></p> <p>I've done some testing for the collision: I've rendered 10,000 random strings a hundred times and checked the uniqueness every time. Done this with length 3, 4 (as you did), 5 and 6. So here are the results:</p> <pre><code>-------- Length 3 ------------ Minimal duplicates: 170 Maximum duplicates: 241 Average duplicates: 207.05 -------- Length 4 ------------ Minimal duplicates: 0 Maximum duplicates: 8 Average duplicates: 3.19 -------- Length 5 ------------ Minimal duplicates: 0 Maximum duplicates: 2 Average duplicates: 0.05 -------- Length 6 ------------ Minimal duplicates: 0 Maximum duplicates: 0 Average duplicates: 0 </code></pre> <p>Absolute 0% chance of collision can't be ruled out as also pointed out by <a href="https://stackoverflow.com/questions/15903606/php-unique-id-within-a-month#comment22648410_15903739">Mark B</a> referring to the Birthday Problem.</p>

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