Add standalone PHP web application to existing site

There is running web site (on Bitrix CMS) located on the simplest shared hosting with cPanel. The task is to create a standalone little web application in PHP (for internal using in the company), which, for simplicity, will be available from the same domain, but, what is important, it will not affect the already running site.
On the hosting, in the file manager there is a public_html folder - as I understand these are the site files. I created in public_html a subfolder (for example, webapp) and added a couple of test files (for example, test.txt). The test file is supposed to be available at site.com/webapp/test.txt, but nothing is loaded - just a white screen, no error messages. Trying with text files and simple php-scripts - the same result, just white screen. How to fix it?