MySQL extension for PHP 5.3 and greater requires that the MySQL server is using a newer and more secure password hash.
If you receive the following error:
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.
You can resolve this by logging into the database using phpMyAdmin or another MySQL client tool with that username and password, selecting the database and running the following SQL command:
SET SESSION old_passwords=FALSE;
SET PASSWORD = PASSWORD('your-existing-password-in-plain-text');