How to integrate with Paypal API in PHP/Cake

Upgrade to a Business Account under Website Payments Pro then "Sign Up" at the bottom
Fill out the form with all the company details, customer service numbers etc.
Once you've been granted business access, your "My Account->Profile" tab in the main screen will change (that should happen instantly after the request.)
Go to "API Access" and you should end up in https://www.paypal.com/uk/cgi-bin/webscr?cmd=_profile-api-access

You might want to work with IPNs (Instant Payment Notifications) to get notified of billings:

Again, click on "My Account->Profile"
On the right column, click on Instant Payment Notifications
Set you URL there, you'll get a HTTP POST to this URL whenever you get a payment or any other notification (subscription cancellation etc.)

Setting up a Sandbox account:

Go to http://developer.paypal.com and sign up for a new account. You can then add as many accounts as you wish.
The accounts will always have the format "chosenbyyou_paypalID_pre/per/biz@..."
you can also set the password, but by default it's the same as the ID

Testing with the Sandbox:

There is a cascaded system of user logins
You have to be logged in as the super user and then log in as sandbox demo account
If you are using a hosted button, you need to create one, one for your live and one for your test account. They will have different IDs.

Receiving IPNs:

When you receive an IPN, you have to verify against the paypal API. Send back all received parameters adding plus

 $transaction['cmd'] = '_notify-validate';

back to PayPal and they will either return

VERIFIED

INVALID