Send partial of FTP stream to php://output - php

Question

I have a PHP-server that serves audio-files by streaming them from an FTP-server not publicly available.
After sending the approriate headers, I just stream the file to the client using ftp_get like this:
ftp_get($conn, 'php://output', $file, FTP_BINARY);
For reasons that has to do with Range headers, I must now offer to only send a part of this stream:
$start = 300; // First byte to stream
$stop = 499; // Last byte to stream (the Content-Length is then $stop-$start+1)
I can do it by downloading the entire content temporarily to a file/memory, then send the desired part to the output. But since the files are large, that solution will cause a delay for the client who has to wait for the file to first be downloaded to the PHP-server before it even starts to download to the client.
Question:
How can I start streaming to php://output from an FTP-server as soon as the first $start bytes have been discarded and stop streaming when I've reached the '$stop' byte?

Answer 1

Instead of using PHP's FTP extension (eg. ftp_get), it is possible to open a stream using PHP's built-in FTP wrapper.
The following code would stream parts of an FTP-file to php://output:
$start = 300; // First byte to stream
$stop = 499; // Last byte to stream
$url = "ftp://username:password#server.com/path/file.mp3";
$ctx = stream_context_create(array('ftp' => array('resume_pos' => $start)));
$fin = fopen($url, 'r', false, $ctx);
$fout = fopen('php://output', 'w');
stream_copy_to_stream($fin, $fout, $stop-$start+1);
fclose($fin);
fclose($fout);
While stream_copy_to_stream has an $offset parameter, using it resulted in an error because the stream was not seekable. Using the context option resume_pos worked fine however.