PHP list all files in directory

Possible Duplicate:
Get the hierarchy of a directory with PHP
Getting the names of all files in a directory with PHP

I have seen functions to list all file in a directory but how can I list all the files in sub-directories too, so it returns an array like?

$files = files("foldername");

So $files is something similar to

array("file.jpg", "blah.word", "name.fileext")