如何从url知道flv大小的大小？

If I know flv video, for example:

www.domain.com/video.flv

I can't use "filesize" because it doesn't able to check size from external url, right?
So, how can I know his size?

You can use CURL to get the headers of a remote file, including the size of a file.

$url = 'http://www.domain.com/video.flv';

$ch = curl_init(); 
curl_setopt($ch, CURLOPT_HEADER, true); 
curl_setopt($ch, CURLOPT_NOBODY, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true); 
curl_setopt($ch, CURLOPT_URL, $url); 
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE); 
$headers = curl_exec($ch);

$size = curl_getinfo($ch, CURLINFO_CONTENT_LENGTH_DOWNLOAD);

curl_close($ch);

Setting CURLOPT_NOBODY to true, makes CURL not download the body of a requested URL.

You could use CURL to send a HTTP HEAD request to the URL you want to know the file size of. The server should send back a content-length header in its response, listing the filesize in octets (bytes).

The server isn't necessarially going to actually send the content-length header though. If it doesn't, then your only option is to actually download the file in full.

I can't use "filesize" because it doesn't able to check size from external url, right?

Yes it can do that, but it's a bit ridiculous to do so, because the url wrappers will first download the file and then check its size.

You could do a HEAD request, which will ask the server for the file size without requesting the file data.