Возвращает TRUE в случае успешного завершения, FALSE в случае возникновения ошибки.
mysql_select_db() sets the current active database on the server that's associated with the specified link identifier. If no link identifier is specified, the last opened link is assumed. If no link is open, the function will try to establish a link as if mysql_connect() was called without arguments, and use it.
Every subsequent call to mysql_query() will be made on the active database.
For downward compatibility mysql_selectdb() can also be used. This is deprecated however.