Processing.js running on a website can only get information that URLs can provide it, and since there are no "dir listings" on the web, it can't grab dir listing content for a URL for you work with. However, depending on what you really want to do, there might be a way to make it work without resorting to PHP.
Assuming you have your Pjs page running on www.example.org/index.html, and you want to list content for www.example.org/sketch/, one option is to simply have a file www.example.org/sketch/list.txt containing all the filenames that the sketch can access, and simply grab that with a
String[] fileNames = loadStrings("./sketch/list.txt")
instruction.
If you can give an example of what you mean with "I need to write an app that first scans the sketch folder to prepare a list of provided files", a more specific solution is probably possible (i.e., what are the files, what does the user need them for, etc)