Weibin Yao,
it's working but may be I'm doing something wrong.
Here is what I do.

$uri_hash = md5('your_secret_word'.'/subdir1/subdir2/movie.flv', true);
$uri_ehash = base64_encode($uri_hash);

this is result of uri_ehash
http://127.0.0.1/subdir1/subdir2/movie.flv?md5=F96syvsfhvPq54ME2VEX4A==

this show me err 403
http://127.0.0.1/subdir1/subdir2/movie.flv

I delete some part of hash

download is complete
1. F96syvsfhvPq54ME2VEX4A==

download is complete
2. F96syvsfhvPq54ME2VEX4A=

download is complete
3. F96syvsfhvPq54ME2VEX4A

this show me err 403
4.F96syvsfhvPq54ME2VEX4

Is this right ?