如何在这个PHP函数中设置比特率来合并音频文件

我正在通过这个函数合并多个音频文件（.wav），但现在我想指定输出文件的比特率。我该怎么办？

function joinwavs($wavs) {
    $fields = join('/', array('H8ChunkID', 'VChunkSize', 'H8Format',
        'H8Subchunk1ID', 'VSubchunk1Size',
        'vAudioFormat', 'vNumChannels', 'VSampleRate',
        'VByteRate', 'vBlockAlign', 'vBitsPerSample'));
    $data = '';
    foreach ($wavs as $wav) {
        $wav = str_replace('http://example.com/', '', $wav);
        $fp = fopen($wav, 'rb');
        $header = fread($fp, 36);
        $info = unpack($fields, $header);
        // read optional extra stuff
        if ($info['Subchunk1Size'] > 16) {
            $header .= fread($fp, ($info['Subchunk1Size'] - 16));
        }
        // read SubChunk2ID
        $header .= fread($fp, 4);
        // read Subchunk2Size
        $size = unpack('vsize', fread($fp, 4));
        $size = $size['size'];
        // read data
        $data .= fread($fp, $size);
    }
    return ($header . pack('V', strlen($data)) . $data);
}