Websites cannot connect to MySQL with one of the following errors:
PLESK_INFO: Your PHP installation appears to be missing the MySQL extension which is required by WordPress
PLESK_INFO: Error displaying the error page: Application Instantiation Error: Could not connect to MySQL.
PHP MySQL extension has been corrupted.
Connect to the server using SSH.
Reinstall PHP MySQL extension:
for RHEL-based :
# yum reinstall php-mysql
for Debian-based :
# apt-get install php5-mysql --reinstall
Note : The package depends on the php version used, e.g