What is the purpose of the PHP mysql_select_db function?

The purpose of the PHP `mysql_select_db` function is to select a MySQL database to work with. This function sets the current active database on the MySQL connection specified by the `link_identifier` parameter. Once a database is selected, all subsequent queries will be executed on that database until a new database is selected with `mysql_select_db` or a new connection is established.

The syntax for the `mysql_select_db` function is as follows:

bool mysql_select_db ( string $database_name [, resource $link_identifier ] )

Here, the `database_name` parameter specifies the name of the database to select, and the optional `link_identifier` parameter is a MySQL connection identifier returned by `mysql_connect`. If the `link_identifier` parameter is not specified, the last opened connection is used.

It is important to note that the `mysql_*` functions are deprecated in PHP 5.5.0 and removed in PHP 7.0.0. It is recommended to use MySQLi or PDO_MySQL extensions instead.