random - PHP code for generating decent-looking coupon codes (mix of letters and numbers)

Question

For an ecommerce site I want to generate a random coupon code that looks better than a randomly generated value. It should be a readable coupon code, all in uppercase with no special characters, only letters (A-Z) and numbers (0-9).

Since people might be reading this out / printing it elsewhere, we need to make this a simple-to-communicate value as well, perhaps 8-10 characters long.

Something like perhaps,

AHS3DJ6BW 
B83JS1HSK

(I typed that, so it's not really that random)

Answer 1

$chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
$res = "";
for ($i = 0; $i < 10; $i++) {
    $res .= $chars[mt_rand(0, strlen($chars)-1)];
}

You can optimize this by preallocating the $res string and caching the result of strlen($chars)-1. This is left as an exercise to the reader, since probably you won't be generating thousands of coupons per second.