How to connect three kinds of databases in PHP __ database

How to connect three kinds of databases in PHP

mysqli
$mysqli = new Mysqli ("example.com", "User", "password", "database");
$result = $mysqli->query ("Select" Hello, dear MySQL user! ') As _message from DUAL ");
$row = $result->fetch_assoc ();
Echo htmlentities ($row [' _message ']);

PDO
$pdo = new PDO (' mysql:host=example.com;dbname=database ', ' user ', ' password ');
$statement = $pdo->query ("Select" Hello, dear MySQL user! ') As _message from DUAL ");
$row = $statement->fetch (PDO::FETCH_ASSOC);
Echo htmlentities ($row [' _message ']);

MySQL
$c = mysql_connect ("example.com", "User", "password");
mysql_select_db ("database");
$result = mysql_query ("Select ' Hello, dear MySQL user! ') As _message from DUAL ");
$row = Mysql_fetch_assoc ($result);
Echo htmlentities ($row [' _message '])

Everyone according to their own needs, you can refer to.