Listing all the folders subfolders and files in a directory using php

Please give me a solution for listing all the folders,subfolders,files in a directory using php. My folder structure is like this:

Main Dir
 Dir1
  SubDir1
   File1
   File2
  SubDir2
   File3
   File4
 Dir2
  SubDir3
   File5
   File6
  SubDir4
   File7
   File8

I want to get the list of all the files inside each folder.

Is there any shell script command in php?