Thanks for you help Ray, I've upgraded to DBv2 and I can connect to php 7.3 databases.
It did take a several attempts though.
I think there was a general caching issue within DW and my computer.
On the first site I tested: From within a MySQLi recordset whenever I clicked test I got the your php server doesn't have the module installed error message. After around a dozen or so attempts what seemed to force fix the error was clicking into the Query Builder advanced section within the WA recordset builder. This seemed to have the effect of pushing the updated _mmServerScripts/MMHTTPDB.php and _mmServerScripts/mysql.php files onto the server.
On the second site I tested, everything connected first time.
I was on the verge of messaging in and asking you to do a remote access to help me fix it but then it connected.