oggvorbis
PHP Manual

Context options

OGG/Vorbis tuning options
Option Definition Relevance Default
pcm_mode PCM byte encoding used. See constants below. Read / Write OGGVORBIS_PCM_S16_LE
rate PCM Sampling rate. Measured in Hz. Write only 44100
bitrate Vorbis Average Bitrate Encoding / Variable Bitrate Encoding. Measured in bps (ABR) or Quality level (VBR: 0.0 to 1.0). 128000 ABR is rough equal to 0.4 VBR. Write only 128000
channels Number of PCM channels. 1 == Mono, 2 == Stereo. Write only 2
serialno Serial Number of stream within file. Must be unique within file. Because of the potential to select a duplicate serial number within a chained file, make efforts to manually assign unique numbers when encoding. Write only Random
comments Associative array of file comments. Will be translated to strtoupper($name) . "=$value". Note: This context option is not available in oggvorbis-0.1 Write only array('ENCODER' => 'PHP/OggVorbis, http://pear.php.net/oggvorbis')

oggvorbis
PHP Manual