致命错误：在null [关闭]上调用成员函数test（）

<?php
class database{
    private static $instance;
    public function __construct()
    {       
    }   
    static public function newinstance()
    {

        if(!self::$instance)
        {
            self::$instance = new self();
        }
        return self::$instance;
    }
    public function conn()
    {
    mysql_connect('localhost','root','') or die("database connection error".mysql_error());
    mysql_select_db('user') or die("cannot select db".mysql_error());

    }
    public function test()
    {
        echo "Public function test";
    }

}
$db=database::newinstance()->conn();
$db->test();

?>

When i rung this php file i got this error:Fatal error: Call to a member function test() on null , what could be the problem?Thank you!

conn() method returns nothing, which you have assigned to $db. Try with:

$db = database::newinstance();
$db->conn();
$db->test();

or you can return $this in conn() method and use your syntax:

public function conn()
{
  mysql_connect('localhost','root','') or die("database connection error".mysql_error());
  mysql_select_db('user') or die("cannot select db".mysql_error());
  return $this;
}

conn() doesn't return anything, so $db is never set to anything. Maybe you meant to do this?:

$db=database::newinstance();
$db->conn();
$db->test();

try this ...

<?php
    class database{
        private static $instance;
        public function __construct()
        {       
        }   
        static public function newinstance()
        {

            if(!self::$instance)
            {
                self::$instance = new self();
            }
            return self::$instance;
        }
        public function conn()
        {
        mysql_connect('localhost','root','') or die("database connection error".mysql_error());
        mysql_select_db('user') or die("cannot select db".mysql_error());

        }
        public function test()
        {
            echo "Public function test";
        }

    }
    $db=database::newinstance();
    $db->conn();
    $db->test();

    ?>