Fatal Error: Contact system adminstrator. Unable to connect to Database : OS Ticket

Fatal Error: Contact system administrator . Unable to connect to Database : OS Ticket

Cause:
You would get this error if your PHP has been upgraded to 5.6 and Later. It happens
due do the Security feature of PHP version 5.6 or later. The error is “PHP Warning:  mysql_connect(): mysqlnd cannot connect to MySQL 4.1+
using the old insecure authentication. Please use an administration tool to reset your password with the command SET PASSWORD = PASSWORD('your_existing_password').
This will store a new, and more secure, hash value in mysql.user.”

Solution:

Check for the ost-config.php file. Find Out the DBUSER and Password from that File.
logging into MySQL as that particular user and running the below commands should resolve the issue;

[phpuser@webserver ~]$ mysql -u <user> -h localhost -p
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 796
Server version: 5.0.95 Source distribution

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> SET SESSION old_passwords = 0;
Query OK, 0 rows affected (0.00 sec)

mysql> SET PASSWORD = PASSWORD('<password>');
Query OK, 0 rows affected (0.00 sec)

mysql> exit
Bye
[phpuser@webserver ~]$

Now update the new password in the config.