找不到“PHPMailer”类

Question

I'm trying to send my mails via phpmailer. So I placed the Exception.php, PHPMailer.php and SMTP.php into a directory called phpmailer.

When I try this php tells me Class 'PHPMailer' not found in /path/to/file.php on line 9

Here my snippet

<?php

ini_set('display_errors', 'on');

require 'phpmailer/Exception.php';
require 'phpmailer/PHPMailer.php';
require 'phpmailer/SMTP.php';

$mail = new PHPMailer(true); 

What is the problem? Why php cant find the PHPMailer class?