PHP/mysqli未定义变量/空属性错误

当试图返回数据库对象$conn时，我一直得到错误"未定义的变量:conn"answers"无法访问空属性"。但是，数据库连接正在工作。是否有更好的方法来返回一个数据库对象比$this->$conn;?

class database {
    private $host = "xxxx";
    private $database = "xxxx";
    private $user = "xxxx";
    private $password = "xxxx";
    public $conn = "";
    function db_connect() {
        try {
            $conn = mysqli_connect($host,$user,$password,$database);
            $conn;
            echo "Connected to database";
            echo "<br>";
            return $conn;
        }
    catch(PDOException $e)
        {
            echo 'Connection Failed:';
            echo "<br>";
            echo $e->getMessage();
        }
}
public function __construct(){
    $this->db_connect();
    return $this->$conn;
}