After a lot of fiddling and fuming, I finally got a valid build of the pgsql client files for PHP7/Apache on -current. There was actually very little work needed for the Slackbuild file to create them, just a lot of bits and pieces that were required to be in place first.

First, you must download and build the PostgreSQL server. I have found no way around this, even though it seems a bit much. I never did create a database cluster nor activate that server, since that is already running on a dedicated box elsewhere on my network. I used 10.10, so that data directory does exist but is still empty.

Next, download the current php-pgsql source archive from the same location as the old version. Then grab the current Slackbuild archive.

Once you unpack the archive, copy the new pgsql source archive into the directory. Then you need to make two changes to the .Slackbuild itself.

1. Change the version string to match your source (-7.4.7).
2. The ext/pgsql/README file no longer exists, so that needs to be removed from the copy line near the bottom. CREDITS is still there, so don't remove the whole line.

Then build and install the SBo as usual.

I now have what appears to be a functional instance of Nextcloud 18.0.6 running on -current with Apache2, PHP7 and an external PostgreSQL server. I still have some adjustments to allow it to fully use that server, but so far everything seems to be working as expected.