仅使用此：
ftp_get($conn_id, "PHP://output", $file, FTP_BINARY);
虽然如果要添加Content-Length标题，则必须先使用来查询文件大小ftp_size：
$conn_id = ftp_connect("ftp.example.com");
ftp_login($conn_id, "username", "password");
ftp_pasv($conn_id, true);
$file_path = "remote/path/file.zip";
$size = ftp_size($conn_id, $file_path);
header("Content-Type: application/octet-stream");
header("Content-Disposition: attachment; filename=" . basename($file_path));
header("Content-Length: $size");
ftp_get($conn_id, "PHP://output", $file_path, FTP_BINARY);
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