PHP Lame stream filter

Question

I have a task to make an mp3 player embedded on a page that will play some voice messages stored in database. Some of messages are stored in WAV-format, so they must be converted to mp3. The conversion should be done "on fly". Because of not all messages have to be converted, I desided that it will be a good idea to use a stream filter that will be used when needed.

class LameFilter extends php_user_filter
{
  protected $process;
  protected $pipes = array();

  public function onCreate() {
    $descriptorspec = array(
       0 => array("pipe", "r"),
       1 => array("pipe", "w"),
       //2 => array("pipe", "w"),
    );

    $this->process = proc_open('lame --cbr -b 128 - -', $descriptorspec, $this->pipes);
  }

  public function filter($in, $out, &$consumed, $closing) {
    while ($bucket = stream_bucket_make_writeable($in)) {

      fwrite($this->pipes[0], $bucket->data);

      $data = '';
      while (true) {
        $line = fread($this->pipes[1], 8192);
        if (strlen($line) == 0) {
            /* EOF */
            break;
        }
        $data .= $line;
      }

      $bucket->data = $data;
      $consumed += $bucket->datalen;
      stream_bucket_append($out, $bucket);
    }
    return PSFS_PASS_ON;
  }

  public function onClose() {
    //$error = stream_get_contents($this->pipes[2]);
    fclose($this->pipes[0]);
    fclose($this->pipes[1]);
    //fclose($this->pipes[2]);
    proc_close($this->process);
  }
}

/* Register our filter with PHP */
stream_filter_register("lame", "LameFilter")
  or die("Failed to register filter");

$mp3 = fopen("result.mp3", "wb");

/* Attach the registered filter to the stream just opened */
stream_filter_append($mp3, "lame");

$wav = fopen('ir_end.wav', 'rb');
while (!feof($wav)) {
  fwrite($mp3, fread($wav, 8192));
}

fclose($wav);
fclose($mp3);

In example I used reading from one file and writing to another one. But actually data is read from OCI-lob and must be written to STDOUT.

The problem is that line "$line = fread($this->pipes[1], 8192);" blocks the script actually independently on expected data length.

Is there any correct way to read from process not closing its STDIN?