I have a client (non hosting) who needs recurring billing for paypal and athorize.net. I already have the billing system set up for them I just need the code that makes it recurring. any help here?

What billing system do you have? Have you created on your own?

Generally, you would need a cron run every day and create the invoice for the billing period. There is no "just the code" for this, it may be simple or very advanced and all depends on the software you are using.

A nice billing system that supports recurring payments is WHMCS - it can be used as a general (non-hosting) solution - it works out of the box, it will take one or two days to setup, configure, test and learn.