There are lots of tutorials showing how to add trailing slashes to a URL with mod_rewrite, but 99.9% of them hard code the domain.
You might want to always append a trailing slash if you depend on it in $_SERVER['REDIRECT_URL'] or if you want to ensure that content is only available via a single URL for search engines etc.
Here’s the Apache mod_rewrite code:
<IfModule mod_rewrite.c>
RewriteEngine On
# Always append a trailing slash
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !/$
RewriteRule . %{REQUEST_URI}/ [R=301,L]
</IfModule>








