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我在php數(shù)據(jù)庫(kù)連接的時(shí)候報(bào)錯(cuò),最后一句顯示錯(cuò)誤“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”

<?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();

最后一句顯示錯(cuò)誤“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”