PHP PNG图像质量50%

我有一个php脚本，我用它来转换上传的图像一半的质量，所以我可以节省服务器空间，但它不工作，我是新的php，所以希望有人能帮助我在哪里我走错了。

if(isset($_FILES['t1']['name'])){
    $file = rand(0, 10000000).$_FILES['t1']['name'];
    if (move_uploaded_file($_FILES['t1']['tmp_name'], $file)) {
        if($fp = fopen($file,"rb", 0))
        {
           $picture = fread($fp,filesize($file));
           fclose($fp);
           $img = imagecreatefrompng($file);
           imagepng($img, $file, 6); //6 quality setting
           imagedestroy($img);
           $tag1 = '<img src="'.$file.'" alt="" class="default" />';
//unlink($file);
echo "<script>$(document).ready(function() {var write = $('.item:nth-child(1)').html();localStorage.item1Pantry = write;});</script>";      
        }
    }

}