RewriteEngine On

# Check if not an existing directory
RewriteCond %{REQUEST_FILENAME} !-d
# Check if appending .php points to an existing file
RewriteCond %{REQUEST_FILENAME}\.php -f
# Rewrite the neat URL to its .php counterpart
RewriteRule ^(.*)$ $1.php [L]

<IfModule mod_headers.c>
    Header set Cache-Control "no-cache, no-store, must-revalidate"
    Header set Expires "0"
</IfModule>