class Database {

    /* Properties */
    private $conn;
    private $dsn = 'mysql:dbname=test;host=127.0.0.1';
    private $user = 'root';
    private $password = '';

    /* Creates database connection */
    public function __construct() {
        try {
            $this->conn = new PDO($this->dsn, $this->user, $this->password);
        } catch (PDOException $e) {
            print "Error!: " . $e->getMessage() . "";
            die();
        }
        return $this->conn;
    }
}

include "database.php";

class User {
    /* Properties */
    private $conn;

    /* Get database access */
    public function __construct() {
        $this->conn = new Database();
    }

    /* Login a user */
    public function login() {
        $stmt = $this->conn->prepare("SELECT username, usermail FROM user");
        if($stmt->execute()) {
            while($rows = $stmt->fetch()) {
                $fetch[] = $rows;
            }
            return $fetch;
        }
        else {
            return false;
        }
    }
}

$user = new User();
$list = $user->login();

foreach($list as $test) {
    echo $test["username"];
}

<?php
$host = '127.0.0.1';
$db   = 'test';
$user = 'root';
$pass = '';
$charset = 'utf8';

$dsn = "mysql:host=$host;dbname=$db;charset=$charset";
$opt = [
    PDO::ATTR_ERRMODE            => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
    PDO::ATTR_EMULATE_PREPARES   => false,
];
$pdo = new PDO($dsn, $user, $pass, $opt);

<?php
class User {
    /* Properties */
    private $conn;

    /* Get database access */
    public function __construct($pdo) {
        $this->conn = $pdo;
    }

    /* List all users */
    public function get_users() {
        return $this->conn->query("SELECT username, usermail FROM user")->fetchAll();
    }
}

include 'database.php';
$user = new User($pdo);
$list = $user->get_users();

foreach($list as $test) {
    echo $test["username"];
}

username_foo
username_bar
username_baz

<?php

class Database {

/* Properties */
private $conn;
private $dsn = 'mysql:dbname=test;host=127.0.0.1';
private $user = 'root';
private $password = '';

/* Creates database connection */
public function __construct() {
}

public function getConnection() {
    try {
        $this->conn = new PDO($this->dsn, $this->user, $this->password);
    } catch (PDOException $e) {
        print "Error!: " . $e->getMessage() . "";
        die();
    }
    return $this->conn;
    }
}
?>