Is MD5 a good way to generate account verification code

Question

When users register an account they get an email with a verification code that they can click to verify their accounts.

This is how I generate the verification code.

md5(rand(0,1000)

Is using the method below a bad choice? It generates a random number between 0-1000. Since there are only 1000 options, and their MD5 hashes are known, it should take an attacker just a 1000 trials to verify the account without it really belonging to them

Answer 1

Just seed it with something the attacker could not know:

md5(rand(0,1000).'helloworld234');

There is no limit at how crasy you could go

md5(md5(time().'helloguys'.rand(0,9999)));

Way too much but you get the idea.

Answer 2

This thread http://stackoverflow.com/questions/46231/how-to-generate-a-verification-code-number has some good thoughts on the matter. Hashes, reversible hashes, check-digits... plenty of options depending on your needs.