Postgresql/PostGIS is a great database. I've used for many years now in conjunction with Recorder.
You can use migration tools to do move the tables across, but then you would have to re-code the stored procedures, triggers, data types etc. Then code the application logic. Its do-able, but given the complex beast that Recorder has become, not trivial.
The other route might be to use FreeTDS to give you connectivity to MS SQL Server from UNIX and other platforms from which you could build your PHP application.
If you really do want to use Postgres, then an option would be to copy or replicate just the tables that form the basic NBN Data model e.g. SURVEY down to TAXON_OCCURRENCE and make, say, SAMPLE the basis of a spatial search table. Then you have to keep the tables synced.
Kind Regards, Dave.
Biodiversity Technology Officer,
Biodiversity Information Service for Powys and Brecon Beacons National Park.