header

header('Content-type: image/png');header('Content-Disposition: attachment; filename="index.png"');readfile('http://beijingmarathon2015.hupu.com/frontSource/beijingMarathon2015/images/index.png');

http://php.net/manual/zh/function.header.php

http://php.net/manual/zh/function.http-send-content-disposition.php