mysql不能使用localhost，怎么回事？

代码如下：

<?php
$mysqli = new mysqli('localhost', 'root', '184995511', 'cg_levi');
if ($mysqli->connect_error) {
    die('Connect Error (' . $mysqli->connect_errno . ') '
            . $mysqli->connect_error);
};

echo 'ok';

查看了hosts没有问题，如下：

127.0.0.1 localhost
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
/etc/hosts (END)

查看mysql状态，没有问题，如下：

mysql> status;
--------------
mysql  Ver 14.14 Distrib 5.6.10, for Linux (x86_64) using  EditLine wrapper

Connection id:      860
Current database:   
Current user:       root@localhost
SSL:            Not in use
Current pager:      stdout
Using outfile:      ''
Using delimiter:    ;
Server version:     5.6.10 MySQL Community Server (GPL)
Protocol version:   10
Connection:     Localhost via UNIX socket
Server characterset:    latin1
Db     characterset:    latin1
Client characterset:    utf8
Conn.  characterset:    utf8
UNIX socket:        /var/lib/mysql/mysql.sock
Uptime:         13 hours 13 min 50 sec

Threads: 1  Questions: 11900  Slow queries: 0  Opens: 100  Flush tables: 1  Open tables: 80  Queries per second avg: 0.249
--------------

请问如何解决？