PHP Mailing List

Does anybody know of a free php mailing list script that automatically sends out emails once someone has signed up.  For example, I have 10 pre-written emails that will go out once per week for 10 weeks for new signups.  So if someone signs up today, they will get email #1 and then next week #2.  But then if someone signs up next week I want them to start from email #1.

So I need an auto-sender that sends in intervals starting from email #1 when someone signs up.  I think I made this sound much more complicated than it really is, but you get the idea!

Thanks!