致命错误：在null [关闭]上调用成员函数test（）

Question

<?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!

Answer 1

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;
}