I've worked with PHP since 2002, and have integrated with PayPal checkout and API on multiple occasions, both through custom cart software and e-commerce frameworks.
It sounds from your project description that you are looking for integrated PayPal checkout, but with the account-free option enabled through your account. That's certainly something I can provide, but I wasn't sure if it also includes shipping or custom fields. I have worked with both, so I can include them as needed.
Thanks!