PHP & Apache with SSL

Hello, any idea how to limit file access (php files) by protocol type call? I mean how could I serve a php file only if it is called with SSL (https://... not http://...) I saw this on a site when I was refused because I didn't use secure protocol, and I wonder, how could I do that? Is there a specific statement in apache config file? Daniel BI PS. I know it 's a little bit far of PHP specific this question, excuse me for that.. maybe you knwow something about