Please note: These are general points, which apply to *any* application that uses a MySQL database. They are not particular to simpleContact.
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MySQL database connection
In the majority of cases, your MySQL database server is accessible through the default setting of "localhost". However some hosts choose to store their databases elsewhere.
On Yahoo! your database host is: "mysql"
You should set the $server (Lite) or $db_host (Pro) setting in your config file to reflect this.
Please note: Do not use the database named mysql for your scripts, as this database is reserved by Yahoo! to administrate your MySQL database environment.
Reference: What are my database name and host name? (Yahoo Small Business Help)