How to get Long 19 digits random numbers at PHP?

So I need to make unique purchase number of mobile game. Each user's buying of in-app-purchase item record should be attached by unique long type 19 digits numbers from php code and stored to mysql DB.

But how can I actually make random 19 digits numbers?

I tried like this but errors.

$num_str = sprintf("%19d", mt_rand(1, 9999999999999999999));