Rewrite friendly URLs (do not mark as a duplicate, please)

I am very confused so I decided to buy support from the Stack Overflow community.

I have a URL shortening service with no database that stores the URLs in the / urls directory and can be accessed as site.com/url, without having to write site.com/urls/url.php and removing the PHP file extension. Which means that when you write site.com/url, you are doing a rewrite of site.com/index.php?url=$1, which in my PHP code includes the file, checking first whether it exists or not.

That is already ready and functional. I use the following code in my .htaccess:

RewriteEngine On
RewriteRule ^(\w+)/?$ index.php?url=$1.php

And I also use the following PHP code in the index.php file so that the rewrite can be carried out correctly:

if($_GET['url']){
$requestedshortenedurl = $_GET['url'];
$urlpath = "urls/$requestedshortenedurl";
if (file_exists($urlpath))
{
    echo "";     
}
else
{
    header("Location: sitio.com?error=4");
    exit;
}

include ("urls/$requestedshortenedurl");
exit;
} else {
    echo "";
}

Now I was looking for how to make site.com?stats=url&visits and site.com?stats=url&visitors can be written as site.com/stats/url/visits and site.com/stats/url/ visitors.

I am using the following code in my .htaccess which is not working:

# Rewrite for shortened URLs visits
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^stats/([^/]+)/([^/]+)/?$ index.php?stats=$1.txt&visits [L]

# Rewrite for shortened URLs visitors
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^stats/([^/]+)/([^/]+)/?$ index.php?stats=$1.txt&visitors [L]

And I'm using the following code in my index.php file:

// Define variable stats
$stats = $_GET['stats'];
// Switch stats cases (visits or visitors)
if($_GET['stats'])
switch($_GET['stats']) {
            case 'visits' :
                if (file_exists($statsvisits))
{
    include ("stats/$statsvisits");     
}
else
{
    header("Location: sitio.com?error=4");
    exit;
}
                break;
            case 'visitors' :
                if (file_exists($statsvisitors))
{
    include ("stats/$statsvisitors");     
}
else
{
    header("Location: sitio.com?error=4");
    exit;
}
                break;
    default :
        header("Location: sitio.com?error=4");
        break;
}

Which is not working and it takes me to sitio.com? error = 4 (404 error page).

The file I'm testing is stpe-visits.txt, which is located at site.com/stats/stpe-visits.txt. When writing site.com/urls/stpe-visits, it does not work.

Thank you.