RewriteEngine On

# remove trailing slash (/) if no such folder exists
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)/$ /${lc:$1}  [NC,L,R=301]

# internally rewrite to a php script if it exists and convert to lower case
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)$ /${lc:$1}.php [NC,L]

# redirect to .php-less link if requested directly
RewriteRule ^(.+)\.php$ ${lc:$1}  [NC,L,R=301]