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 n

Answer 1

$size = 12;

$string = strtoupper(substr(md5(time().rand(10000,99999)), 0, $size));