submit_paypal_post() is a pretty rancid way of doing what you want it to do.
validate_ipn could check if there are things like curl, instead of fputs.
eregi is deprecated.
Overall this class is decent if you aren't an experienced programmer and want some help. If you really know what you are doing, and want up-to-date code, I suggest you write your own class.
The documentation is abundant and pretty good.