1. Install remi repo in centos 8
dnf install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm

dnf install https://rpms.remirepo.net/enterprise/remi-release-8.rpm
  1. Install php 5.6 and its required modules
yum install php56.x86_64
  1. Register the php.56 fcgi module with php, If all good you will get handler id and successfully registered message.
/usr/local/psa/bin/php_handler --add -displayname php-56 -path /usr/bin/php56-cgi -phpini /etc/opt/remi/php56/php.ini -clipath /usr/bin/php56 -type fastcgi

The new PHP handler with the id "150e2a3d0f6ca2bb60a26372169d231c3c758e6d" was successfully registered.

  1. You can verify it using the below command
plesk bin php_handler --list |grep 5.6
  1. Now the php version will be listed in front end of plesk in php version drop down

ที่มา : https://talk.plesk.com/threads/how-to-install-centos-8-plesk-5-php-version.358438/