下列常量由此扩展定义，且仅在此扩展编译入 PHP 或在运行时动态载入时可用。

YAC_VERSION (string)

YAC_MAX_KEY_LEN (int)

Max length of a key could be, it is 48 bytes.

YAC_MAX_VALUE_RAW_LEN (int)

YAC_MAX_RAW_COMPRESSED_LEN (int)

YAC_SERIALIZER_PHP (int)

Use php serialize as serializer

YAC_SERIALIZER_JSON (int)

Use json as serializer(requrie --enable-json)

YAC_SERIALIZER_IGBINARY (int)

Use igbinary as serializer(require --enable-igbinary)

YAC_SERIALIZER_MSGPACK (int)

Use msgpack as serializer(require --enable-msgpack)

YAC_SERIALIZER (string)

Which serialzier is yac used