Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
Two things would help narrow this down. What exactly is in the PHP test file? And when it fails, do you see your browser’s own “HTTP ERROR 500” page, or are you redirected to an InfinityFree error page at infinityfree.net/errors/500/?
The two point in different directions. Our knowledge base says the redirect to the InfinityFree 500 page usually means broken .htaccess rules. The plain browser “HTTP ERROR 500” usually means the PHP code crashed. Renaming the root .htaccess and still getting the 500 doesn’t settle it, because the test file could be crashing for its own reasons. If you can, try a file containing only <?php echo "hello"; and tell me what it returns.
Here’s the article on the browser’s HTTP ERROR 500, including how to get PHP to show the real error:
And here’s the one on the InfinityFree 500 error page:
Your account looks active and was created a few days ago, so nothing there stands out. I haven’t found anything yet that explains a 500 on a plain PHP file. If the minimal file also fails and there are no other .htaccess files in htdocs or its subfolders, that may need a person to look at it.