There is a known issue with the pdo_oci extension which was not loading correctly.
It can be identified by the following error when executing the php -v command:
PHP Warning: PHP Startup: Unable to load dynamic library 'pdo_oci.so' (tried: /opt/alt/php**/usr/lib64/php/modules/pdo_oci.so (libclntsh.so.12.1: cannot open shared object file: No such file or directory), /opt/alt/php**/usr/lib64/php/modules/pdo_oci.so.so (/opt/alt/php**/usr/lib64/php/modules/pdo_oci.so.so: cannot open shared object file: No such file or directory)) in Unknown on line 0
- PHP Selector
1. Install the missing libclntsh.so.12.1 library with the following command:
yum install oracle-instantclient12.1-basic
The libclntsh.so.12.1 library was missing.