PHP - 仅显示目录和子目录中的PHP文件

This is my first project using PHP and I've been stuck on this for almost 2 days now! I want to get all my PHP files from a directory and any sub directories and display them in iframes. I've managed to get it to display all the files in the directory, but I only want the PHP files, please see below. Thanks in advance for any help/advice.This is my first project using PHP and I've bee