演示代码报错？
private function _isUsernameExists($username){
$sql = 'SELECT * FROM `user` WHERE `username`=:username';
$stmt = $this->_db->prepare($sql);
$stmt ->bindParam(':username',$username);
$stmt ->execute();
$result = $stmt->fetch(PDO::FETCH_ASSOC);
return !empty($result);
}$stmt = $this->_db->prepare($sql);这行代码一直报错？
