php to append emails to text file

[ice9]

I'm looking for a simple modification to register.php that will append the email addresses of newly registered users to a text file for addition to my mailing list (lyris). I looked through the hack archives, and found some other more robust mailing list features, but I'm looking for something a lot simpler. I'm coming from a perl background, and am relatively new to php.

I just need a plain checkbox on my registration form "check here to receive our newsletters, etc.", that will append the email to a text file. Then, I'll just cron something nightly to add the email addresses to my lyris list. I don't need to check it against a list to see if they are already subscribed.