The only way to force Guest Checkout (which is what PayPal calls it when somebody uses a credit card from their checkout pages without signing in to a PayPal account) is to use the Express Checkout APIs.
This will allow you to set parameters that provide the Guest Checkout experience in all instances where it's available. Keep in mind that even with this configured correctly, Guest Checkout is simply not available for all buyers in all countries, so you still won't get it for 100% of your customers if you're selling world wide.
Guest Checkout is available in PayPal Standard Payments, too, but it's cookie based, so it's very unreliable. If the browser in use has ever had anybody sign in to a PayPal account then it will assume the current user will be signing in, too, and it will not prominently display the Guest Checkout option. This is what you are able to avoid by using the Express Checkout APIs.
This PayPal PHP SDK will make the Express Checkout calls very quick and easy for you. Specifically, you'll be working with:
In your SetExpressCheckout request you'll make sure to include the follow parameters:
- SOLUTIONTYPE=Sole
- LANDINGPAGE=Billing
- USERSELECTEDFUNDINGSOURCE=CreditCard
This will ensure that the user sees the Guest Checkout experience.