向所有帐户添加管理员访问权限

我想知道如何实现这一点，或者这是否可能。我有一个应用程序，它不是由管理员创建的，可以访问所有用户帐户，但现在我希望它可以访问所有帐户。

谢谢。（尽管我还没有尝试过任何东西，只是不知道如何开始，我也不想重新开始：建议）

我当前的登录代码是：

        if(isset($_POST['login'])){
        $username = stripslashes($_POST['username']);
        $password = stripslashes($_POST['password']);

        $stmt = $pdo->prepare("SELECT password FROM tablename WHERE username=:username");
        $stmt->bindValue(':username', $username, PDO::PARAM_STR);
        $stmt->execute();
        if($stmt->rowCount()<1){
        echo '<div class="signals"><p class="bg-warning text-center warning"><button type="button" class="close" aria-label="Close"><span aria-hidden="true">&times;</span></button>INVALID USERNAME OR PASSWORD</div></p>';
        }else{
        list($hash) = $stmt->fetch(PDO::FETCH_NUM);
        if (password_verify($password, $hash)) {
        //$_SESSION['username'] = $username;
        $status1 = "COMPLETED";
        $status2 = "PROCESSING";
        //$stmt = $pdo->query("SELECT status FROM ca_confirmed WHERE username ='$_SESSION[username]'");
        $stmt = $pdo->query("SELECT status FROM tablename WHERE username ='$username'");
        $check = $stmt->fetch(PDO::FETCH_ASSOC);
        $status = $check['status'];
        $_SESSION['username'] = $username;
        if(strcmp($status, $status1) == 0){
        header("location: completed/index.php");
        exit();
        }elseif(strcmp($status, $status2) == 0){
        header("location: uncompleted/index.php");    
        //exit();
        }
        }else{
        echo '<div class="signals"><p class="bg-warning text-center warning"><button type="button" class="close" aria-label="Close"><span aria-hidden="true">&times;</span></button>INVALID USERNAME OR PASSWORD again</div></p>';
        }    
        }
        }