<?php
class Db{
	static private $_instance;
	static private $_connectSource;
	private $_dbConfig = array(
		'host' => '127.0.0.1',
		'user' => 'root',
		'password' => '',
		'database' => 'stage',
	);
	private function __construct(){
		
	}
	
	static public function getInstance(){
		if(self::$_instance instanceof self){
			self::$_instance = new self();
		}
		return self::$_instance;
		
	}
	public function connect(){
		if(!self::$_connectSource){
		self::$_connectSource = mysqli_connect($this->_dbConfig['host'],$this->_dbConfig['user'],$this->_dbConfig['password']);
		
		if(!self::$_connectSource){
			die('mysql connect error'.mysql_error());
		}
		mysqli_select_db(self::$_connectSource,$this->_dbConfig['database']);
		mysqli_query(self::$_connectSource,"set names UTF8");
		echo "success";
		}
		return self::$_connectSource;
		
	}
}
$connect = Db::getInstance() -> connect();

最后一句显示错误“Fatal error: Uncaught Error: Call to a member function connect() on null in /Applications/XAMPP/xamppfiles/htdocs/connect.php:38 Stack trace: #0 {main} thrown in /Applications/XAMPP/xamppfiles/htdocs/connect.php on line 38”