php 发送邮件乱码

php发送邮箱乱码 ：（缺少加红部分）
$mail = new PHPMailer();
$mail->IsSMTP();
$mail->SMTPAuth = true;
$mail->Host = $smtp;
$mail->Username = $youremail;
$mail->Password = $password; //必填， 以上邮箱对应的密码
$mail->From = $youremail;
$mail->FromName = $site_name; //发件人称呼
$mail->AddAddress($ymail,$yname);
$mail->CharSet = "utf-8";