All answers are now outdated. Latest versions (as of February 2018) no longer have autoloading, PHPMailer should be initialized as follows:

<?php

  require("/home/site/libs/PHPMailer-master/src/PHPMailer.php");
  require("/home/site/libs/PHPMailer-master/src/SMTP.php");

    $mail = new PHPMailer\PHPMailer\PHPMailer();
    $mail->IsSMTP(); // 启用SMTP

    $mail->SMTPDebug = 1; // 调试：1 = 错误和消息，2 = 仅消息
    $mail->SMTPAuth = true; // 启用身份验证
    $mail->SMTPSecure = 'ssl'; // 启用安全传输，Gmail要求
    $mail->Host = "smtp.gmail.com";
    $mail->Port = 465; // 或者587
    $mail->IsHTML(true);
    $mail->Username = "xxxxxx";
    $mail->Password = "xxxx";
    $mail->SetFrom("xxxxxx@xxxxx.com");
    $mail->Subject = "测试";
    $mail->Body = "你好";
    $mail->AddAddress("xxxxxx@xxxxx.com");

     if(!$mail->Send()) {
        echo "邮件发送失败：" . $mail->ErrorInfo;
     } else {
        echo "邮件已发送";
     }
?>