Just require a file, or check if_readable first?
I don't recommend B:


A file being readable does not guarantee that it can be successfully included.  The specified path could be a directory (weird and unlikely, but possible), or it might point to a file with a syntax error.
is_readable() does not check the include_path the way require_once would.


Supposing that you could write a function to ensure that it is a regular file, and that it does not contain syntax errors, and checked all directories in the search path, you would have written a significant amount of code that reimplements PHP's built-in functionality.  The built-in functionality isn't broken to begin with.  If you say that a file is required, and PHP is unable to include it, that's a fatal error.  What is the point of prettifying a fatal error and suppressing the error code?

If the file to be loaded is not entirely essential, or if you want your script to commit suicide on its own terms, a better strategy might be to check the return value of include_once.