Resumable downloads when using PHP to send the file?

The first thing you need to do is to send the Accept-Ranges: bytes header in all responses, to tell the client that you support partial content. Then, if request with a Range: bytes=x-y header is received (with x and y being numbers) you parse the range the client is requesting, open the file as usual, seek x bytes ahead and send the next y – x bytes. Also set the response to HTTP/1.0 206 Partial Content.

Without having tested anything, this could work, more or less:

$filesize = filesize($file);

$offset = 0;
$length = $filesize;

if ( isset($_SERVER['HTTP_RANGE']) ) {
    // if the HTTP_RANGE header is set we're dealing with partial content

    $partialContent = true;

    // find the requested range
    // this might be too simplistic, apparently the client can request
    // multiple ranges, which can become pretty complex, so ignore it for now
    preg_match('/bytes=(\d+)-(\d+)?/', $_SERVER['HTTP_RANGE'], $matches);

    $offset = intval($matches[1]);
    $length = intval($matches[2]) - $offset;
} else {
    $partialContent = false;
}

$file = fopen($file, 'r');

// seek to the requested offset, this is 0 if it's not a partial content request
fseek($file, $offset);

$data = fread($file, $length);

fclose($file);

if ( $partialContent ) {
    // output the right headers for partial content

    header('HTTP/1.1 206 Partial Content');

    header('Content-Range: bytes ' . $offset . '-' . ($offset + $length) . "https://stackoverflow.com/" . $filesize);
}

// output the regular HTTP headers
header('Content-Type: ' . $ctype);
header('Content-Length: ' . $filesize);
header('Content-Disposition: attachment; filename="' . $fileName . '"');
header('Accept-Ranges: bytes');

// don't forget to send the data too
print($data);

I may have missed something obvious, and I have most definitely ignored some potential sources of errors, but it should be a start.

There’s a description of partial content here and I found some info on partial content on the documentation page for fread.