<?PHP
class DB{

//set the connection property to private to prevent direct access 
private static $conn;

//now since we dont want to reinstiate the class anytime we need it,lets also set the constructor to private 
private function __construct(){}

//now lets create our method for connecting to the database 
public static function connect(){

//now lets check if a connection exists already in our $conn property,then we should return it instead of recreating a new connection 
if(!empty(self::$conn)){
return self::$conn;
}//end if 

//upon reaching here means the $conn property is empty so lets create a new connection 

try {
 $dbh = new PDO('MysqL:host=127.0.0.1;dbname=lingtong',array(PDO::ATTR_PERSISTENT => true));

//lets now assign the database connection to our $conn property 
self::$conn = $dbh;

//return the connection 
return $dbh;

} catch (PDOException $e) {
 print "Error! : " . $e->getMessage() . "<br/>";
 die();
}

}//end method 

}//end class

?>

我们的单例类只能创建一个连接并重用它,让我们看看如何使用我们的类

<?PHP 
$dbh = DB::connect();

foreach ($dbh->query('SELECT * from agent') as $row){ 
  print_r($row);
}
?>