Unable connect to local MySQL server through socket '/tmp/mysql.sock' (2) while trying to install application on Plesk for Linux

Symptoms

1. Installation of application in Domains > example.com > Website & Settings > Applications > Install application or an attempt to access WordPress Toolkit page fails with the error:

   PLESK_INFO: Exception: ODBC error #08S01: [unixODBC][MySQL][ODBC 5.1 Driver]Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)
   file: /usr/share/aps_php/aps_php.php
   line: 12519
   code: 0

2. The same error is shown while accessing Plesk > Tools & Settings > Application Vault

3. MySQL service is running.

Cause

The /tmp/mysql.sock link does not exist.

Resolution

1. Connect to the server via SSH
2. Check /tmp/mysql.sock link file:

   # ls -l /tmp/mysql.sock
   ls: cannot access /tmp/mysql.sock: No such file or directory

3. Create the link if it does not exist:

   # ln -s /var/lib/mysql/mysql.sock /tmp/mysql.sock