使调整大小的图像透明

我使用此代码将图像上传到服务器。但由于某种原因，我的缩略图变成了黑色背景，而不是原来的透明。

我试过使用这个"指南"：http://www.akemapa.com/2008/07/10/php-gd-resize-transparent-image-png-gif/

但没有成功。我不知道如何将这段代码实现到我使用的代码中。

如果有人能帮忙，请这样做：）

 <?php
    //define a maxim size for the uploaded images
    define ("MAX_SIZE","10240"); 
    // define the width and height for the thumbnail
   // note that theese dimmensions are considered the maximum dimmension and are not fixed, 
   // because we have to keep the image ratio intact or it will be deformed
 define ("WIDTH","200"); 
 define ("HEIGHT","200"); 
  // this is the function that will create the thumbnail image from the uploaded image
 // the resize will be done considering the width and height defined, but without deforming the image
 function make_thumb($img_name,$filename,$new_w,$new_h)
 {
    //get image extension.
    $ext=getExtension($img_name);
    //creates the new image using the appropriate function from gd library
    if(!strcmp("jpg",$ext) || !strcmp("jpeg",$ext))
        $src_img=imagecreatefromjpeg($img_name);
    if(!strcmp("png",$ext))
        $src_img=imagecreatefrompng($img_name);
    if(!strcmp("gif",$ext))
        $src_img=imagecreatefromgif($img_name);
        //gets the dimmensions of the image
    $old_x=imageSX($src_img);
    $old_y=imageSY($src_img);
     // next we will calculate the new dimmensions for the thumbnail image
    // the next steps will be taken: 
    //  1. calculate the ratio by dividing the old dimmensions with the new ones
    //  2. if the ratio for the width is higher, the width will remain the one define in WIDTH variable
    //      and the height will be calculated so the image ratio will not change
    //  3. otherwise we will use the height ratio for the image
    // as a result, only one of the dimmensions will be from the fixed ones
    $ratio1=$old_x/$new_w;
    $ratio2=$old_y/$new_h;
    if($ratio1>$ratio2) {
        $thumb_w=$new_w;
        $thumb_h=$old_y/$ratio1;
    }
    else    {
        $thumb_h=$new_h;
        $thumb_w=$old_x/$ratio2;
    }
    // we create a new image with the new dimmensions
    $dst_img=ImageCreateTrueColor($thumb_w,$thumb_h);
    // resize the big image to the new created one  
    imagecopyresampled($dst_img,$src_img,0,0,0,0,$thumb_w,$thumb_h,$old_x,$old_y); 
    // output the created image to the file. Now we will have the thumbnail into the file named by $filename
    if(!strcmp("png",$ext))
        imagepng($dst_img,$filename);
    else
        imagejpeg($dst_img,$filename);
        imagegif($dst_img,$filename); 
    //destroys source and destination images. 
    imagedestroy($dst_img); 
    imagedestroy($src_img); 
 }
 // This function reads the extension of the file. 
 // It is used to determine if the file is an image by checking the extension. 
 function getExtension($str) {
         $i = strrpos($str,".");
         if (!$i) { return ""; }
         $l = strlen($str) - $i;
         $ext = substr($str,$i+1,$l);
         return $ext;
 }
// This variable is used as a flag. The value is initialized with 0 (meaning no error found) and it will be changed to 1 if an errro occures. If the error occures the file will not be uploaded.
 $errors=0;
 // checks if the form has been submitted
 if(isset($_POST['Submit']))
 {
 //reads the name of the file the user submitted for uploading
    $image=$_FILES['image']['name'];
    // if it is not empty
    if ($image) 
    {
        // get the original name of the file from the clients machine
        $filename = stripslashes($_FILES['image']['name']);
        // get the extension of the file in a lower case format
        $extension = getExtension($filename);
        $extension = strtolower($extension);
        // if it is not a known extension, we will suppose it is an error, print an error message and will not upload the file, otherwise we continue
        if (($extension != "jpg")  && ($extension != "jpeg") && ($extension != "png") && ($extension != "gif")) 
        {
            echo '<h1>Unknown extension!</h1>';
            $errors=1;
        }
        else
        {
            // get the size of the image in bytes
            // $_FILES[''image''][''tmp_name''] is the temporary filename of the file in which 
            //the uploaded file was stored on the server
            $size=getimagesize($_FILES['image']['tmp_name']);
            $sizekb=filesize($_FILES['image']['tmp_name']);
            //compare the size with the maxim size we defined and print error if bigger
            if ($sizekb > MAX_SIZE*10240)
            {
                echo '<h1>You have exceeded the size limit!</h1>';
                $errors=1;
            }
            //we will give an unique name, for example the time in unix time format
            $image_name=time().'.'.$extension;
            //the new name will be containing the full path where will be stored (images folder)
            $newname="../images/uploads/".$image_name;
            $copied = copy($_FILES['image']['tmp_name'], $newname);
            //we verify if the image has been uploaded, and print error instead
            if (!$copied) 
            {
                echo '<h1>Copy unsuccessfull!</h1>';
                $errors=1;
            }
            else
            {
                // the new thumbnail image will be placed in images/thumbs/ folder
                $thumb_name='../images/uploads/thumbs/thumb_'.$image_name;
                // call the function that will create the thumbnail. The function will get as parameters the image name, the thumbnail name and the width and height desired for the thumbnail
                $thumb=make_thumb($newname,$thumb_name,WIDTH,HEIGHT);
            }}  }}