PHP Warning: mysqli_connect(): (HY000/2002): Connection refused

In case anyone else comes by this issue, the default port on MAMP for mysql is 8889, but the port that php expects to use for mysql is 3306. So you need to open MAMP, go to preferences, and change the MAMP mysql port to 3306, then restart the mysql server. Now the connection should be successful with host=localhost, user=root, pass=root.

Sometimes you need to include mysql db port id in the server like so.

$serverName = "127.0.0.1:3307";