使用PHP确定是否已替换远程文件？

Question

I have a MySQL database with some URLs in it. One URL per row. Each URL has my script on it. What I am wanting to do, is check if the file is still there via a PHP script. Not check if it 404'd, but rather check if it has been modified or replaced. Is this possible? If so, how would it be accomplished?

I was thinking making the remote file echo some string, and having the local file check the page for that string, but that seems a little inefficient and sloppy.

EDIT: The remote files don't output any data to the remote users, they trigger internal scripts, so remote users would only see a white page. (The checking script would only see this as well)

Answer 1

Bulletproof Way

1. Fetch the URL contents.
2. Compute the MD5() or SHA1() hash.
3. Fetch the URL again and check if the hash has changed.

There are other ways (such as the Last-Modified HTTP header), but they don't work with all servers.