php生成唯一的码

function make_coupon_card() {
$code = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
$rand = $code[rand(0,25)]
.strtoupper(dechex(date('m')))
.date('d').substr(time(),-5)
.substr(microtime(),2,5)
.sprintf('%02d',rand(0,99));
for(
$a = md5( $rand, true ),
$s = '0123456789ABCDEFGHIJKLMNOPQRSTUV',
$d = '',
$f = 0;
$f < 8;//设置码的个数
$g = ord( $a[ $f ] ),
$d .= $s[ ( $g ^ ord( $a[ $f + 8 ] ) ) - $g & 0x1F ],
$f++
);
return $d;
}