I believe he was asking if file_exists() checks files relative to $PHP_DOCUMENT_ROOT rather than having to specify the absolute directory on the server.
In fact, include() and require() do not read files relative to $PHP_DOCUMENT_ROOT. The paths searched for by these functions is specified by the "include_path" directive in the PHP configuration script. The default for the parameter is "./" which means your scripts check their current directories first.
You shoud be able to specify the file in file_exists() relative to the current directory or to $PHP_DOCUMENT_ROOT.
You say you want to change the root of a "module". Are talking about a compile-time module or just a library you are going to include() in your pages?
-- rich --