Hi.
You can use the
mt_rand function to generate a random number. You can even specify the lowest and highest limits.
As to using a single random number for the entire month. You will obviously have to store this number somewhere, where it can be read by your code. I would probably use a Database, like MySQL.
You would simply have to have a field for your random number, and a date field for the month it represents. If there is not a field for the current month, you use the function to generate a new number and add it for the current month.