This probably has nothing to do with performance but...
I have created many PORTABLE php class files (like win32 dlls) which contain functions that I can use in different web applications. The funnctions I have created are well coded and I have made sure that they can be used in all necessary ways.
Regarding performance, provided the server is an up-to-date machine, you are unlikely to be able to notice any difference in the performance if the functions are put in a different php file or contained within the same one.
For the sake of editing i personally would create different files for different sets of functions but comment them well for future editors (including yourself) to enable a greater understanding of what is going on. I have before coded php applications and not commented them and got completely lost in how they are working.
Hope this post interests you if not helps you.
Thanks
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