<?php
class Db{
    private static $conn;

    const HOST     = 'localhost';
    const DB       = 'tudo';
    const CHARSET  = 'utf8';
    const USER     = 'root';
    const PASSWORD = '';

    public static function conn(){
		if(!self::$conn):
			try{
				self::$conn = new \PDO('mysql:host='.self::HOST.';dbname='.self::DB.';charset='.self::CHARSET.';',self::USER,self::PASSWORD);
			}catch(\PDOException $e){
	            echo $e->getMessage();
	        }
		endif;
		return self::$conn;
	}
}

$conn_1 = Db::conn();
$conn_2 = Db::conn();

var_dump($conn_1);
echo '<br>';
var_dump($conn_2);

# Output:
#object(PDO)#1 (0) { }
#object(PDO)#1 (0) { }