php<?php
//function.php
function &connectdb()
{
    global $DBname,$DBusername,$DBpassword,$servername;
    $conn=new mysqli($servername, $DBusername, $DBpassword, $DBname);
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    }
    $conn->query("SET NAMES 'UTF8'");
    return $conn;
}
?>

php<?php
/**
*
*/
class Bar
{
    private $conn;
    function __construct()
    {
        require './function.php';
        $this->conn=connectdb();
        $result=$this->conn->query("SELECT * FROM 15_spring_new WHERE 1;");
        var_dump($this->conn->error);
    }
}
$bar = new Bar;
 ?>

发现只要在函数中调用，就会出现这个问题。
该错误输出为："No database selected"
而当我在类外面使用时，程序运行正常。
错误在哪里？