Ok - discovered the solution in pgsql-php, repeated below for reference:
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From: "Peter De Muer (Work)" <peter.demuer@inter-networks.be>
To: <pgsql-php@postgresql.org>
Subject: Re: 7.3.1 update gives PHP libpq.so.2 problem
Date: Tue, 4 Feb 2003 14:06:04 +0100
try making a soft link libpq.so.2 to the libpq.so.3 file that comes
with
PHP 7.3.1.
regards,
pt3r
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End of quoted message
So (for me) its a case of:
cd /usr/lib
ln -s libpq.so.3 libpq.so.2
