try this
RewriteEngine On
# Unless directory, remove trailing slash
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/$ http://example.com/folder/$1 [R=301,L]
# Redirect external .php requests to extensionless url
RewriteCond %{THE_REQUEST} ^(.+)\.php([#?][^\ ]*)?\ HTTP/
RewriteRule ^(.+)\.php$ http://example.com/folder/$1 [R=301,L]
# Resolve .php file for extensionless php urls
RewriteRule ^([^/.]+)$ $1.php [L]