Warning: mysqli_connect(): Unknown MySQL server host

The port number must be a separate argument:

$link = mysqli_connect('host', 'user', 'pass', 'db', 5306);

Any ideas on what to do?

Sure. A simple three-step solution for any php function's problem:

1. Open your favorite browser
2. type php.net/ in the address bar followed by a problem function's name:
   `php.net/mysqli_connect` in your case
3. Hit Enter

Now you have the function's description and can check the proper parameters list.