I use AirAlliance datebase for testing. And this sample.

1. Create php file. press Alt+Ins.Select Connection to Database ...
2. Select jdbc:mysql://localhost:3306/AirAlliance and press OK

Result:
$conn2 = mysqli_connect('localhost:3306', 'root', 'adminadmin', 'AirAlliance');
    if (!$conn2) {
        die('Could not connect to MySQL: ' . mysqli_connect_error());
    }
    mysqli_query($conn2, 'SET NAMES \'UTF-8\'');
    // TODO: insert your code here.
    mysqli_close($conn2);

This code does not work. If change 'localhost:3306' to 'localhost'
See there is 'localhost:3306' value in  mysqli_connect procedure


mysqli_connect  function has this Description:
mysqli mysqli_connect  ([ string $host  [, string $username  [, string $passwd  [, string $dbname  [, int $port  [,
string $socket  ]]]]]] )

NB
invoke
mysqli_connect('localhost', 'root', 'adminadmin', 'AirAlliance', 3306);

it works.