Random activeation code

[Anon]

Ok I would like to generate a random code of which I can attach to a email for the users to active there account with in PHP.

I have got everything else working but I am looking for some coding of which would gernate a 20 characther code of which considers of letters (upper and lower) and also numbers too.

Anybody know how I can do that?

[Anon]

$source="abcdefghijklmnopqrstuvwxyzQWERTYUIOPASDFGHJKLZXCVBNM0123456789";
$sourcelen=strlen($source);
$activationcode='';

// seed with microseconds since last "whole" second
srand ((double) microtime() * 1000000);
for($i=0;$i<20;++$i)
{
$activationcode.=$source[rand(0,$sourcelen-1)];
}
echo $activationcode;

[Anon]

Because this uses the microsecond feature, is it safe to say that the $activationcode will be truly unique?

[Anon]

As random as PHP's Mersenne twister algorithm can make it - since microtime returns a Unix timestamp, to within a measured microsecond, the seed given is doesn't repeat between 1970-01-01 and 2038-01-18 (unless there are two hits that are recorded as happening during the same millionth of a second).

[Anon]

If you want to get fancy with your seed you can combine the user's IP address (if available) with the microtime, throw in a few other bits of entropy, then shovel the whole lot through an MD5 and read the first 32 bits of that as your seed integer.

$ip=ip2long($REMOTE_ADDR);
mt_srand((double)microtime()*1000000+$ip);
$seed = hexdec(substr(md5(uniqid(mt_rand(),true)),0,16));

But how careful do you really need to be?

[Anon]

No I won't be needing that level of uniqueness. Your previous method is perfect, thanks.