Tersus is not limited to HSQLDB, and works equally well with any of the supported databases (see http://www.tersus.com/#Id=45 for a list, and http://www.tersus.com/#Id=242 + http://www.tersus.com/#Id=376 for configuration details).
As for PHP supporting HSQLDB, although it does not appear in the link you supplied, doing a quick search for the two, did find a few pages discussing the issue and providing information.
One thing to note with regards to Tersus' use of HSQLDB is that by default, it accesses the database in 'In-Process (Standalone) mode' (see hsqldb documentation @ http://hsqldb.sourceforge.net/doc/guide/ch01.html#N1013D) which keeps the database files locked from access by other tools concurrently. If you do wish to have concurrent access from Tersus and other tools, and still use HSQLDB, you will need to configure HSQLDB to run in Server mode (see the same link).
I hope this gives you some pointers forward, and please post further questions if they arise.
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