<?php echo date("Ymd") ?>
$userID= date('Ymd') + some number
Not to say you are doing it wrong, but you should consider using the AUTO_INCREMENT property built into the DB. This will give you a integer number that automatically increases with each insert.
You can set it as an Attribute of the table,
And then when doing inserts you just leave that field out, and magically it will be incremented. If you need to use it immediately after doing the insert you can get its value with ( in MySqli )
mysqli_insert_id( $db );
Then for the created time, you should use a proper data type such as a timestamp or datetime. This too can be set to have a default value of
NOW() the current time. Then when you do date based queries you can use things like
MONTH( create_date ) to find all users that singed up in a given month etc..
Here is a wiki page on what I meant by Surrogate key
Basically it's just a unique identifier not derived from the actual data. Then being these are users a good example of a Natural key would be the email address, being that users email should be unique in most cases.