In a .htaccess file in your document root
Options +FollowSymLinks
RewriteEngine On
# Remove .php.
RewriteCond %{ENV:REDIRECT_STATUS} ^$
RewriteCond %{SCRIPT_FILENAME} -f
RewriteRule ^(.+)\.php$ /$1/ [R=301,L]
# Add .php back.
RewriteCond %{DOCUMENT_ROOT}/$1.php -f
RewriteRule ^(.*[^/])/?$ /$1.php [QSA,L]