How to protect non-WordPress files in a subdirectory of your site using Paid Memberships Pro.

my_pmpro_getfile.php

<?php
/*
  This code handles loading a file from the /protected-directory/ directory.

  (!) Be sure to change line 44 below to point to your protected directory if something other than /protected/
  (!) Be sure to change line 64 below to check the levels you need.
  (!) Add this code to your active theme's functions.php or a custom plugin.
  (!) You should have a corresponding bit of code in your Apache .htaccess file to redirect files to this script. e.g.

  ###
  # BEGIN protected folder lock down
  <IfModule mod_rewrite.c>
  RewriteBase /
  RewriteRule ^protected-directory/(.*)$ /?pmpro_getfile=$1 [L]
  </IfModule>
  # END protected folder lock down
  ###
*/
function my_pmpro_getfile()
{
	if(isset($_REQUEST['pmpro_getfile']))
	{
		global $wpdb;
	
		//prevent loops when redirecting to .php files
		if(!empty($_REQUEST['noloop']))
		{
			status_header( 500 );
			die("This file cannot be loaded through the get file script.");
		}
	
		$uri = $_REQUEST['pmpro_getfile'];
		if($uri[0] == "/")
			$uri = substr($uri, 1, strlen($uri) - 1);

		/*
		Remove ../-like strings from the URI.
		Actually removes any combination of two or more ., /, and \.
		This will prevent traversal attacks and loading hidden files.
		*/
		$uri = preg_replace("/[\.\/\\\\]{2,}/", "", $uri);
	
		//edit to point at your protected directory
		$new_uri = "protected-directory/" . $uri;
	
		$filename = ABSPATH . $new_uri;
		$pathParts = pathinfo($filename);				
		
		//remove params from the end
		if(strpos($filename, "?") !== false)
		{
			$parts = explode("?", $filename);
			$filename = $parts[0];
		}
		
		//add index.html if this is a directory
		if(is_dir($filename))
			$filename .= "index.html";
				
		//only checking if the file is pulled from outside the admin
		if(!is_admin())
		{			
			//non-members don't have access (checks for level 2 or 3)
			if(!pmpro_hasMembershipLevel())
			{
				//nope				
				//header('HTTP/1.1 503 Service Unavailable', true, 503);
				//echo "HTTP/1.1 503 Service Unavailable";
				wp_redirect(wp_login_url());
				exit;
			}			
		}
		
		//get mimetype
		require_once(PMPRO_DIR . '/classes/class.mimetype.php');
		$mimetype = new pmpro_mimetype();       		
		$file_mimetype = $mimetype->getType($filename);
		
		//in case we want to do something else with the file
		do_action("pmpro_getfile_before_readfile", $filename, $file_mimetype);
		
		//if file is not found, die
		if(!file_exists($filename))
		{
			status_header( 404 );
	        nocache_headers();        
	        die("File not found.");
		}
		
		//if blacklistsed file type, redirect to it instead
		$basename = basename($filename);
		$parts = explode('.', $basename);
		$ext = strtolower($parts[count($parts)-1]);
		
		//build blacklist and allow for filtering
		$blacklist = array("inc", "php", "php3", "php4", "php5", "phps", "phtml");
		$blacklist = apply_filters("pmpro_getfile_extension_blacklist", $blacklist);
		
		//check
		if(in_array($ext, $blacklist))
		{		
			//add a noloop param to avoid infinite loops
			$uri = add_query_arg("noloop", 1, $uri);
			
			//guess scheme and add host back to uri
			if(is_ssl())
				$uri = "https://" . $_SERVER['HTTP_HOST'] . "/" . $uri;
			else
				$uri = "http://" . $_SERVER['HTTP_HOST'] . "/" . $uri;
					
			wp_redirect($uri);
			exit;
		}
			
		require_once(PMPRO_DIR . '/classes/class.mimetype.php');
		
		//okay show the file
		header("Content-type: " . $file_mimetype); 	
		readfile($filename);
		exit;
	}
}
add_action("init", "my_pmpro_getfile");

Forked here for updates: https://gist.github.com/ideadude/137bb0d06f8f49fc262c2fad3010c440

Since we changed the strangerstudios GitHub account to an organization, I lost the ability to edit old gists.