我的代码是这样的：

<?php
$url = $_GET['img'];  
$filename = basename($url);
$img = grabImage($url,$filename);
echo '<img src="'.$img.'">';
function grabImage($url,$filename){
if ($url == "") return false;
ob_start(); 
readfile($url); 
$img = ob_get_contents(); 
ob_end_clean();
$fp = @fopen($filename,"a"); 
fwrite($fp,$img);
fclose($fp); 
return $filename;
}
?>

然后访问http://test.com/?img=https://scontent.cdninstagram.com/t51.2885-15/s640x640/sh0.08/e35/12950259_974006612694869_608041607_n.jpg出现链接被重置，该怎么改？使用表单传递url可以正常下载。