If you know for sure you are only including a script ONCE, why
would you use require_once? The whole idea of using require_once
is when you dont know if and when you gonna need the script.

When testing performance you test suite should ofcourse include
same files multiple times right? Someone may argue that if your
code is loading same files more than once, then it is a failure
by design. I dont agree. For the same reason we use #ifndef in
our C or C++ programs =)

An example:
Assume I have a utlity class with common stuff and it is used by
both DB.php and Auth.php. Most of the time these two are NOT
loaded in the same script but what if they do? Like the login
script which uses DB backend. Using plain “include” or “require”
will throw fatals if and when both DB and Auth are used in a
script: Cannot redeclare bla bla…

So there is only 2 options left: include_once or __autoload.

Depending on your code I would say:

- If you know your code is using the included file then require_once
is better.

- If the required file is most likely NOT used, then use __autoload

- plain require/include should be used only when the included
file does not contain any PHP code at all. Means you are loading
static content or templates or whatever similar.