pg_connect: Call to undefined function pg_connect()

I have moved a number of php scripts from PHP 7.2.9 to PHP 8.0.27 and I am encountering the
following error:
[05-Dec-2023 15:34:41 America/Toronto] PHP Fatal error:  Uncaught Error: Call to undefined function
pg_connect() in /usr/httpd/xxx0002.php:211

line 211, which worked as expected up until now is:
  $db_handle = pg_connect('dbname=xxxx user=xxxx password=xxxx');
and the online docs show this as still acceptable but deprecated.  I noticed that the result will be
a "PgSql\Connection" object.

Two questions:
1. since the syntax is still valid why am I getting an undefined function error?

2. since the result is different, am I going to encounter a lot of other problems in this conversion
and where can I find a full list of these changes?

Thanks,

John
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