I have 5-10 files on one server that all catch XMLRPC-PHP requests.  There's about 20 lines of require_once in each one that are all the same.  It'd make life simpler for me to create a separate file that only has the requires and require that in my XMLRPC files.  Is this best practice, or does it open up issues that I don't know about?
Is it best practice to have a file only of includes/requires?
php;mvc
I've done that before and it did save some effort, but got it fiddly as I didn't always need every single file and sometimes needed a few more. If you always need those and only those, then I think it's a good plan. However, there are alternatives if you have control of the file hierarchy and are not using third party libraries e.g. using php's __autoload() function to have the files automatically included when you try to instantiate the class. I'd tend to prefer this if at all possible as it needs a lot less maintenance.