This error was driving me crazy:
1.20.2 installation : DB connection error: Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) (localhost).
Check the host, username and password and try again
because everything was correct, and there was nothing in the Apache error_log.
Turns out it was a selinux issue. I was able to connect fine after disabling selinux.