建立数据库连接时出错 - 期望参数2为资源

Question

Reading the log:

PHP Warning: mysql_query() expects parameter 2 to be resource, boolean given in [...] mysql.php on line 14, 15, 16, 17

These:

    mysql_query("SET NAMES 'utf8'", $this->link);
    mysql_query("SET CHARACTER SET utf8", $this->link);
    mysql_query("SET CHARACTER_SET_CONNECTION=utf8", $this->link);
    mysql_query("SET SQL_MODE = ''", $this->link);

It happened out of nowhere, noone was working on that wordpress site. Wp-config.php is ok with user, pass etc The error (in the title) is the same on front-end and back-end.

Any idea, I red a lot of other posts about but nothing seems to help in this case.