How do I use the Paypal API? I'm wanting to do some custom PHP stuff with Paypal. Don't want users to have to login when the payment is submitted as long as they provide their info or something. Is this possible?

Did you read any of the documentation?

You don't need to use PayPal API for this. You just need to enable the Account Optional setting in your PayPal profile. With Account Optional enabled, visitors can pay through credit card without needing to signup or login.

Hope this helps.

There's a topic regarding Paypal IPN on this forum. You can use a very adaptable script, under PHP, in order to integrate it into your current script:

http://www.micahcarrick.com/v2/content/view/1/22/

It's quite easy to use. ;)