include_once("mime.inc");

$mm = new MIME();

$to = "customer@263.net";

$subject = $mm->encode("商城","gb2312"); // 编码

$msg = "注册会员成功
";

$msg .= "会员名：wayne
";

$msg .= "密码：wayne";

$headers = "From: webmaster@263.net\r\nContent-Type: text/html; charset=gb2312\r\n";

mail("$to", "$subject", "$msg", "$headers");

?>

进行编码以后，中文问题解决。