直接看代码吧，很好明白的
复制代码 代码如下:
//用法 echo remote_filesize($url,$user='',$pw='');
$url = "http://www.aa.com/librarys/images/random/rand_11.jpg";//这里要换成你的图片地址
echo remote_filesize($url,$user='',$pw='');

function remote_filesize($uri,$user='',$pw='')
{
// start output buffering
    ob_start();
// initialize curl with given uri
    $ch = curl_init($uri); // make sure we get the header
    curl_setopt($ch, CURLOPT_HEADER, 1); // make it a http HEAD request
    curl_setopt($ch, CURLOPT_NOBODY, 1); // if auth is needed, do it here
    if (!empty($user) && !empty($pw))
    {
        $headers = array('Authorization: Basic ' . base64_encode($user.':'.$pw));
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    }
    $okay = curl_exec($ch);
    curl_close($ch); // get the output buffer
    $head = ob_get_contents(); // clean the output buffer and return to previous // buffer settings
    ob_end_clean();  // gets you the numeric value from the Content-Length // field in the http header
    $regex = '/Content-Length:\s([0-9].+?)\s/';
    $count = preg_match($regex, $head, $matches);  // if there was a Content-Length field, its value // will now be in $matches[1]
    if (isset($matches[1]))
    {
        $size = $matches[1];
    }
    else
    {
        $size = 'unknown';
    }
    $last_mb = round($size/(1024*1024),3);
 $last_kb = round($size/1024,3);
    return $last_kb . 'KB / ' . $last_mb.' MB';
}

函数的思路是，先CURL获取图片到缓冲区，然后正则获取图片的Content-Length信息就OK了。