To reproduce, try to download a file when running Apache http/2. Not that downloads seem to hang randomly.

Summary

The code at components/download/download.php:241 uses readfile() to stream files through PHP, which is incompatible with Apache's mod_http2 due to its response buffering behavior.

Root Cause Analysis

Looking at the output() method (lines 232-248):

  private function output($file, $type)
  {
      switch ($type) {
          case 'data':
              echo $file;
              break;
          case 'file':
              readfile($file);  // <-- This is the problem
              break;
          case 'xsendfile':
              $this->setHeader('X-Sendfile', $file);
              break;
      }
  }

Why this fails with HTTP/2:

• mod_http2 buffers the entire PHP response before transmitting to properly frame HTTP/2 streams
• readfile() sends data through PHP's output buffer, which mod_http2 waits to complete
• Browsers using HTTP/2 see the download stall; wget/curl work because they default to HTTP/1.1

Good News

The code already supports X-Sendfile (xsendfile mode on line 244-245), which completely bypasses this issue by having Apache serve the file directly instead of PHP.

Recommended Fixes

Option 1: Output buffer management (partial fix)

Add before readfile():

  while (ob_get_level()) {
      ob_end_clean();
  }
  if (function_exists('apache_setenv')) {
      apache_setenv('no-gzip', '1');
  }
  @ini_set('zlib.output_compression', 'Off');
  flush();

This helps but may not fully resolve mod_http2 buffering.

Option 2: Promote X-Sendfile usage (recommended)

Document and encourage enabling mod_xsendfile for production environments with HTTP/2. This is the cleanest solution since:

• Apache handles the file transfer directly
• Bypasses PHP output buffering entirely
• More memory-efficient for large files

Option 3: Chunked reading with flushing

Replace readfile($file) with:

  $handle = fopen($file, 'rb');
  while (!feof($handle)) {
      echo fread($handle, 8192);
      ob_flush();
      flush();
  }
  fclose($handle);

This is still susceptible to mod_http2 buffering but may help in some configurations.