下载在电脑端是这样实现的，但在手机端不能用
ob_start();
$this->image();
$s = ob_get_clean();
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-Type: application/force-download");
//header("Content-type:application/octet-stream;");
header("Accept-Ranges:bytes");
header("Content-Type:text/html;charset=utf-8");
header('Content-Length: '.strlen($s));
$ua = $_SERVER["HTTP_USER_AGENT"];
$filename = “abc.jpg";
$encoded_filename = urlencode($filename);
$encoded_filename = str_replace("+", "%20", $encoded_filename);
if (preg_match("/MSIE/", $ua)) {
header('Content-Disposition: attachment; filename="' . $encoded_filename . '"');
} else if (preg_match("/Firefox/", $ua)) {
header('Content-Disposition: attachment; filename*="utf8\'\'' . $filename . '"');
} else {
header('Content-Disposition: attachment; filename="' . $filename . '"');
}
header("Content-Transfer-Encoding: binary");
echo $s;
