How to keep the ?error on url if page extension not included?

I was able to remove the .php or .html extension on my url e.g "localhost/index.php" to "localhost/index" instead. But, on pages with forms, when redirecting back to the form page with the header('Location: /index.php?error=whatever); when there are errors, the ?error=whatever is not included. Like, before it would be "localhost/index.php?error=whatever", but now since I removed the page extension, it just returns to "localhost/index". The error message is not showing up. What I have only done to remove the page extension is via the .htaccess with this code

#turn on url rewriting 
RewriteEngine on
#remove the need for .php extention 
RewriteCond %{REQUEST_FILENAME} !-d 
RewriteCond %{REQUEST_FILENAME}\.php -f 
RewriteRule ^(.*)$ $1.php

What do you guys think I should or could do to fix this?