Aspetta, i tuoi file sono
admin/index.php
config/config.php
function.php
Senza /, giusto? Altrimenti sarebbero nella root ....

Comunque, a me funziona. Vedi http://php.net/manual/en/function.include.php
Files are included based on the file path given or, if none is given, the include_path specified. If the file isn't found in the include_path, include will finally check in the calling script's own directory and the current working directory before failing. The include construct will emit a warning if it cannot find a file; this is different behavior from require, which will emit a fatal error.