mysqli使用localhost问题

<?php
$mysqli = new mysqli('localhost', 'root', '123456', 'mysql');
if ($mysqli->connect_error) {
    die('Connect Error (' . $mysqli->connect_errno . ') '
            . $mysqli->connect_error);
};

echo 'ok';

• 如果上面连接地址为'localhost'就会报错，如下：

  Warning: mysqli::mysqli(): (HY000/2002): No such file or directory in /mnt/www/cglevi/publichtml/mysql.php on line 2 Connect Error (2002) No such file or directory

• 将'localhost'修改为'127.0.0.1'之后链接正常

• 将代码：

  $con = mysqli_connect("localhost","root","123456","mysql");

  修改为：

  $con = mysqli_connect("127.0.0.1","root","123456","mysql");

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
--------------

开始的回答有点不严谨，估计也没有解决问题，修改了答案：

问题出现的原因：

当主机填写为localhost时MySQL会采用 unix domain socket连接，当主机填写为127.0.0.1时MySQL会采用TCP/IP的方式连接。使用Unix socket的连接比TCP/IP的连接更加快速与安全。这是MySQL连接的特性，可以参考官方文档的说明4.2.2.
Connecting to the MySQL Server：

On Unix, MySQL programs treat the host name localhost specially, in a way that is
likely different from what you expect compared to other network-based programs.
For connections to localhost, MySQL programs attempt to connect to the local server
by using a Unix socket file. This occurs even if a --port or -P option is given to
specify a port number. To ensure that the client makes a TCP/IP connection to the
local server, use --host or -h to specify a host name value of 127.0.0.1, or the IP
address or name of the local server. You can also specify the connection protocol
explicitly, even for localhost, by using the --protocol=TCP option.

这个问题有以下几种解决方法：

1. 使用TCP/IP代替Unix socket。即在连接的时候将localhost换成127.0.0.1。

2. 修改MySQL的配置文件my.cnf，指定mysql.socket的位置：

   /var/lib/mysql/mysql.sock (你的mysql.socket路径)。

3. 直接在php建立连接的时候指定my.socket的位置（官方文档：mysqli_connect）。比如：

   $db = new MySQLi('localhost', 'root', 'root', 'my_db', '3306', '/var/run/mysqld/mysqld.sock')

其实答案写的不是很严谨，不过既然被采纳了，就多说一点。 通常意义上localhost和127.0.0.1是等价的，只是mysql在处理这个名词的问题上有一些不同，是根据不同的地址来采取的不同的通信手段。

原因呢，我猜大概是为了本地应用能获得更好的性能。而且localhost这个地址在mysql中也不会做匹配。即user@'%'不能匹配到user@'localhost'

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