服务器中mysql启动正常，但是程序无法连接

Warning: mysql_error(): supplied argument is not a valid MySQL-Link resource in /usr/local/httpd/htdocs/area/include/db_mysql.php on line 299

Warning: mysql_errno(): supplied argument is not a valid MySQL-Link resource in /usr/local/httpd/htdocs/area/include/db_mysql.php on line 303

Error: Can not connect to MySQL server

Error: 

问题解决方法：

我仅仅将host＝localhost更改成host＝127.0.0.1 就可以用，不知为什么？