<?php
    class Database_connection {

        private $db_host =  "local";
        private $db_name = "root";
        private $db_user = "user";
        private $db_pass = "pass";
        protected $db_holder;

        protected function open_connection() {
            $this->db_holder = new PDO($this->db_host.$this->db_name, $this->db_user, $this->db_pass);
        }

        protected function close_connection() {
            $this->db_holder = null;
        }

    }
    ?>

new PDO('mysql:host=localhost;dbname=base', $usuario, $senha);

new PDO($this->db_host

new PDO('mysql:host='. $this->db_host .';dbname='.$this->db_name, $this->db_user, $this->db_pass);