RewriteEngine on

Options -Indexes
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !yoursite.com/a_tools\.php [NC]
RewriteCond %{HTTP_REFERER} !^http://yoursite.com/backup/.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com/backup/.*$ [NC]
RewriteRule .*\.(php|PHP|sql|SQL)$ - [F,NC]