没有加"@"的错误提示为：

Warning: mysql_connect(): [2002] No such file or directory (trying to connect via unix:///tmp/mysql.sock) in ./index.php on line 2

Warning: mysql_connect(): No such file or directory in ./index.php on line 2
出错了，错误原因是：mysql_connect(): No such file or directory

当我们不想让用户看到这样的错误信息，我们用"@"来将错误信息屏蔽掉：

出错了，错误原因是：mysql_connect(): No such file or directory