yii项目连接多个数据库时, MySQL报错No such file or directory

服务器错误

yii\db\Exception
SQLSTATE[HY000] [2002] No such file or directory
###'xxx是项目根目录'
#0 xxx/vendor/yiisoft/yii2/db/Connection.php(928): yii\db\Connection->open()
#1 xxx/vendor/yiisoft/yii2/db/Connection.php(915): yii\db\Connection->getMasterPdo()
#2 xxx/vendor/yiisoft/yii2/db/Schema.php(483): yii\db\Connection->getSlavePdo()
#3 xxx/vendor/yiisoft/yii2/db/Connection.php(824): yii\db\Schema->quoteValue('1')
#4 xxx/vendor/yiisoft/yii2/db/Command.php(175): yii\db\Connection->quoteValue('1')
#5 xxx/vendor/yiisoft/yii2/db/Command.php(866): yii\db\Command->getRawSql()
#6 xxx/vendor/yiisoft/yii2/db/Command.php(887): yii\db\Command->logQuery('yii\\db\\Command:...')
#7 xxx/vendor/yiisoft/yii2/db/Command.php(376): yii\db\Command->queryInternal('fetch', NULL)
#8 xxx/vendor/yiisoft/yii2/db/Query.php(253): yii\db\Command->queryOne()
#9 xxx/vendor/yiisoft/yii2/db/ActiveQuery.php(294): yii\db\Query->one(NULL)
#10 xxx/modules/course/controllers/CourseController.php(249): yii\db\ActiveQuery->one()
#11 [internal function]: app\modules\course\controllers\CourseController->actionSubscribe('1')
#12 xxx/vendor/yiisoft/yii2/base/InlineAction.php(57): call_user_func_array(Array, Array)
#13 xxx/vendor/yiisoft/yii2/base/Controller.php(156): yii\base\InlineAction->runWithParams(Array)
#14 xxx/vendor/yiisoft/yii2/base/Module.php(523): yii\base\Controller->runAction('subscribe', Array)
#15 xxx/vendor/yiisoft/yii2/web/Application.php(102): yii\base\Module->runAction('course/course/s...', Array)
#16 xxx/vendor/yiisoft/yii2/base/Application.php(380): yii\web\Application->handleRequest(Object(app\hejiang\Request))
#17 xxx/web/index.php(7): yii\base\Application->run()
#18 {main}

本地开发测试的时候好好的, 放到线上服务器 就出现问题. 把MySQL用户名, 密码都改成了线上的, 其他的都正常访问, 一遇到连接数据库的地方就会报上面的错误.  最终排查出问题原因: 吧线上服务器的localhost改成127.0.0.1就OK了,本地是可以使用localhost的,线上不行.