paypal now has a all-in-one payment processing interface, which means you can handle credit cards without even bouncing to the paypal website. i’m amazed this didn’t happen ten years ago — the existing schemes with distinct merchant accounts and gateways has always been dodgy.
you do have to use their express checkout thing (which does bounce to them, and lets users use their paypal account directly) in order to use the direct payment api. it’s $20/month and 2.2-2.9% + 30¢ per credit card transaction or 1.9% + 30¢ per paypal payment.
the best part may be that paypal is a company that seems to know where its towel is. what i’ve often heard from people working with existing providers is that they’re either morons or crooks. (or in the case of verisign, both.)
Express checkout does bounce people to PayPal, but PayPal bounces them back to your site after they've signed in and picked their payment source, where you can complete the transaction.
Unlike earlier versions of PayPal, you're not forced to specify a fixed payment amount when sending people to PayPal. Instead you get a payment authorization token, which you can then use to fulfill the transaction.
BTW, there's a PayPal PHP SDK, so it should be easy to use from PHP. I haven't had a chance to play with it yet, but give me a few weeks.