MySQL error

Here is a list of the most common error messages concerning your database:

Access denied for user user@domain

This means that you are using an incorrect login and/or password and/or an incorrect database server address.
Solution: Check your database connection settings from your customer area.

Can't connect to local MySQL server through socket

This means that you did not set the database server address correctly in your connection script. Solution: Check your database connection settings from your customer area.

User user reached max_user_connection limit

This error is due to the fact that you have reached the maximum allowed number of simultaneous connections to your database.
If you have a lot of traffic, it is better to open the SQL connection only when you are going to use it and then close it (with mysql_close()) once you no longer need to interact with the database.
Also, persistent connections (made with mysql_pconnect()) should be avoided.