数据库连接，用于告诉mysql_query函数使用哪个mysql连接执行sql语句。官方文档中的描述：The MySQL connection. If the link identifier is not specified, the last link opened by mysql_connect() is assumed. If no such link is found, it will try to create one as if mysql_connect() had been called with no arguments. If no connection is found or established, an E_WARNING level error is generated.