To get UTF-8 charset you can specify that in the DSN.

$link = new PDO("mysql:host=localhost;dbname=DB;charset=UTF8");

To specify a database connection port use the following DSN string

<?php
$dsn = 'mysql:dbname=testdb;host=127.0.0.1;port=3333';
?>

A generic pattern to connect to a mariadb or mysql database using a settings file

<?php
$_SETTINGS = parse_ini_file('./settings.ini', true);

$db = new \PDO(
  "mysql:hostname={$_SETTINGS['db']['host']};dbname={$_SETTINGS['db']['name']}",
  $_SETTINGS['db']['user'],
  $_SETTINGS['db']['pass'],
  [
    \PDO::ATTR_DEFAULT_FETCH_MODE => \PDO::FETCH_ASSOC,
    \PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8mb4'"
  ]
);
?>
You might not need the options depicted here but I find them convenient. 

Works with a settings.ini file containing for instance:
[db]
host = "localhost"
name = "dbname"
user = "dbuser"
pass = "dbpassword"

To connect throught unix socket you need to use 
<?php
$dsn = 'mysql:dbname=testdb;unix_socket=/path/to/socket';
?>

You musn't specify host when using socket.

When trying to connect to a local database, it seems "uri:file:///" is no longer needed. Just write the PDO pilot name followed by the absolute path of the file.
Example : sqlite:C:\VirtualHosts\phpliteadmin\dbs\surveillance_logeas.s3DB

I'd like to point out that in PHP 7.0 in the dsn parameter you can't use 'host=localhost' to solve this you can use 'host=127.0.0.1' instead.

Most of the information in the comment here is outdated or wrong.

You do can use host=localhost to connect via socket, which is faster than TCP, so setting 127.0.0.1 is a performance loss.

To use proper utf you should use utf8mb4, for example:

$db = new PDO('mysql:host=' . DATABASE_HOST . ';dbname='. DATABASE_NAME .';charset=utf8mb4', DATABASE_USER, DATABASE_PASSWORD);

If you use the UTF-8 encoding, you have to use the fourth parameter :

<?php
$db = new PDO('mysql:host=myhost;dbname=mydb', 'login', 'password', array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES \'UTF8\''));
?>

If you override PDO with own class and you want to implement alias from php.ini, you have to get it with `get_cfg_var` instead of `ini_get`.

<?php
class PDO extends \PDO
{
    public function __construct(string $dsn, ?string $username = null, ?string $password = null, ?array $options = null)
    {
        //alias
        if (!str_contains($dsn, ':')) {
            $dsn = get_cfg_var('pdo.dsn.' . $dsn);
            if (!$dsn) {
                throw new PDOException('Argument #1 ($dsn) must be a valid data source name');
            }
        }

        // your additional logic

        parent::__construct($dsn, $username, $password, $options);
    }
}
?>