下载时传输fopen字节

How to check how much the user has downloaded:

$fp=fopen("$file","rb");

while(!feof($fp))
{
print(fread($fp,1048));
$bytes_transferred = '' // ???

$bytes_transferred = 0;
$fp = fopen($file, 'rb');
if($fp) {
    while(!feof($fp)) {
        $data = fread($fp, 1024); // or can be a little larger, say: 4096, etc
        $len = strlen($data);
        $bytes_transferred += $len;
        echo $data;
    }
    fclose($fp);
}