You need to assign it to a variable first; like so:
$folder = "def_folder";
$inc_folders = array(
'default' => $folder,
'file' => $folder . "/textfile.txt"
);
Or alternatively, build the array in two steps:
$inc_folders = array(
'default' => "def_folder"
);
$inc_folders['file'] = $inc_folders['default'] . "/textfile.txt";