From the website:
QuaaxTM is a PHP Topic Maps engine which supports ISO/IEC 13250-2 Topic Maps Data Model (TMDM). QuaaxTM is an implementation of the core and index interfaces of PHPTMAPI. PHPTMAPI is based on the TMAPI specification and provides a standardized API for PHP5 to access and process data held in a topic map.
QuaaxTM uses MySQL with InnoDB as storage engine and therefore benefits from transaction support and referential integrity.