Hello,
This has nothing to do with the PHP version:) We support all versions starting from 5.3 and up to 7.
Are you able to manually connect to the Kaltura DB from the shell with these credentials? I would assume not..
I suggest you run:
# /opt/kaltura/bin/kaltura-drop-db.sh
which would drop all existing Kaltura DBs and then, manually rerun the DB creation script with:
# bash -x /opt/kaltura/bin/kaltura-db-config.sh
And look closely at the debug info to see where we're failing. Once you have more info, please post it here and I'll be happy to help you further.