Media and file delivery system (in-house CDN)

// Re-sending the same index overwrites the part, so a retry is idempotent.
$request->file('chunk')->move($dir, sprintf('%06d.part', $index));

// ... later, in complete(): assemble the parts in order
$out = fopen($assembled, 'wb');

for ($i = 0; $i < $info['total_chunks']; $i++) {
    $part = $dir.'/'.sprintf('%06d.part', $i);

    if (! is_file($part)) {
        fclose($out);
        return response()->json(['success' => false, 'message' => "Missing chunk: {$i}"], 422);
    }

    $in = fopen($part, 'rb');
    stream_copy_to_stream($in, $out);
    fclose($in);
}
fclose($out);

if (filesize($assembled) !== $info['size']) {
    File::deleteDirectory($dir);
    return response()->json(['success' => false, 'message' => 'Size mismatch.'], 422);
}