Thank you all for your replies. As it turned out, going to PHP 5 was all it took. I love my host, by the way: Site5. They've been running PHP 4 and 5 in parallel for a while--soon to convert over to all PHP 5. All I needed was a directive in my .htaccess file... Come to think of it, I'm glad this issue came up, since I'll be wanting to use PHP 5's XML features on a project coming up.

I've just carried an order all the way through to PayPal successfully. However, one more minor issue has come up. On the "Review Order" screen, the message "You must select a shipping option before continuing" was at the top of the screen--in spite of my having already selected a shipping option which was calculated into the order just fine.