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                **單例模式:在開發中保證某個類的對象實例只能有一個** ## 需要知識: 1. 訪問權限 2. 繼承 3. static 4. 構造函數 5. 對象克隆 6. final關鍵字 ~~~ <?php class DBHelper{ private $conn; private static $instance=null; //防止用戶直接new對象 private function __construct(){ $this->conn=@mysql_connect('localhost','root','root'); } public function query($sql){ echo "完成對數據庫的操作"; } public static function getDB(){ //第一種寫法 //if(self::$instance == null){ //self::$instance=new DBHelper(); //} //第二種寫法 if(!(self::$instance instanceof self)){ self::$instance =new self(); } return self::$instance; } //阻止克隆 private function __clone(){} } $db1=DBHelper::getDB(); $db2=DBHelper::getDB(); echo '<pre>'; if($db1===$db2){ echo "db1===db2"; }else{ echo 'db1和db2不是同一個對象'; } ~~~
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