.htaccess URL rewriting with end-slash

I have the following in my .htaccess file, and it removes the .php of the URL for 'friendly urls'.

Code:

---

        RewriteEngine on
        RewriteCond %{REQUEST_URI} !(\.[^./]+)$
        RewriteCond %{REQUEST_fileNAME} !-d
        RewriteCond %{REQUEST_fileNAME} !-f
        RewriteRule (.*) /$1.php [L]
        RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^.]+\.)+php\ HTTP
        RewriteRule ^(.+)\.php$ /$1 [R=301,L]

---

How do I modify it so that it puts a slash at the end?

Here's an example:
Original: /about.php
Rewritten: /about/

Thanks for you help!

// X96 \\