与localhost的连接有效，但不是ip

Question

If I connect to the database using 'localhost' as hostname, It works

$flag = [
'database_host' => 'localhost',
'database_port' => '',
'database_name' => 'dbdemo',
'database_user' => 'userdemo',
'database_password' => 'uijk',
'database_table' => 'tabledemo',
];

$link = mysqli_connect($flag['database_host'],$flag['database_user'],$flag['database_password'],$flag['database_name']);

But when I set up the hostname by using his ip (which I get through echo $_SERVER['SERVER_ADDR'];), It returns the error

Connection refused

I'm confused, is it not basicaly the same thing?