I am looking for a good tutorial or book that explains how to process a customer's credit card from a form on your site. I read something about using curl, but it wasn't a complete tutorial. I am looking for a lesson that will take me from step 1 to actually charging a customer's credit card.
What is a good tutorial or book to learn credit card processing using PHP?
When you set up a merchant account with an online payment gateway they provide you with complete instructions on how to interface with it and usually some sample code.
Payment gateway integration varies from provider to provider. There are many providers like paypal, authorize.net, CCAvenue, 2CO, etc. So there's no definite universal way of integrate any and all such providers in general. So you need to first conclude on which provider you wish to use and then study integration at respective provider's developer center website.
Once you get to integrate your first provider successfully, you will find subsequent provider integrations very easy.
Here are some references for you to start with.