PHP图片上传:透明度显示为黑色

我对PHP非常陌生，我真的不知道如何修复一个常见的错误。当我使用以下代码上传png时，背景显示为黑色。有一些解决方案显示如何修复它，但我不知道把它放在代码的哪里，我发现的一切都打破了它。我使用以下代码:

<?php
############ Configuration ##############
$destination_folder     = '../img/'; //upload directory ends with / (slash)
##########################################
//continue only if $_POST is set and it is a Ajax request
if(isset($_POST) && isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest'){

$whichimage = $_POST["whichimage"];
// check $_FILES['ImageFile'] not empty
if(!isset($_FILES['image_file']) || !is_uploaded_file($_FILES['image_file']['tmp_name'])){
        die('Image file is Missing!'); // output error when above checks fail.
}
//uploaded file info we need to proceed
$image_name = $_FILES['image_file']['name']; //file name
$image_temp = $_FILES['image_file']['tmp_name']; //file temp
$image_size_info    = getimagesize($image_temp); //get image size
if($image_size_info){
    $image_type         = $image_size_info['mime']; //image type
}else{
    die("Make sure image file is valid!");
}
//switch statement below checks allowed image type
//as well as creates new image from given file
switch($image_type){
    case 'image/png':
        $image_res =  imagecreatefrompng($image_temp); break;
    case 'image/gif':
        $image_res =  imagecreatefromgif($image_temp); break;
    case 'image/jpeg': case 'image/pjpeg':
        $image_res = imagecreatefromjpeg($image_temp); break;
    default:
        $image_res = false;
}
if($image_res){
    //Get file extension and name to construct new file name
    $image_info = pathinfo($image_name);
    $image_extension = strtolower($image_info["extension"]); //image extension
    $image_name_only = strtolower($image_info["filename"]);//file name only, no extension
    //create a name for new image (Eg: fileName_293749.jpg) ;
    $new_file_name = $whichimage . '.' . $image_extension;
    //folder path to save resized images and thumbnails
    $image_save_folder  = $destination_folder . $new_file_name;
    //call normal_resize_image() function to proportionally resize image
    if(normal_resize_image($image_res, $image_save_folder, $image_type))
    {
    }
    imagedestroy($image_res); //freeup memory
}
}
#####  This function will proportionally resize image #####
function normal_resize_image($source, $destination, $image_type){
//do not resize if image is smaller than max size
    if(save_image($source, $destination, $image_type)){
        return true;
    }

}
##### Saves image resource to file #####
function save_image($source, $destination, $image_type){
switch(strtolower($image_type)){//determine mime type
    case 'image/png':
        imagepng($source, $destination); return true; //save png file
        break;
    case 'image/gif':
        imagegif($source, $destination); return true; //save gif file
        break;
    case 'image/jpeg': case 'image/pjpeg':
        imagejpeg($source, $destination); return true; //save jpeg file
        break;
    default: return false;
}
}

这是本教程的修改和精简版本

我不想重新调整图像的大小和改变质量，所以我尝试删除这些部分的代码。

在教程的评论部分，虽然有一些关于修复透明度的帖子:

imagealphablending($NewCanves, false);
imagesavealpha($NewCanves,true);
$transparent = imagecolorallocatealpha($NewCanves, 255, 255, 255, 127);
imagefilledrectangle($NewCanves, 0, 0, $NewWidth, $NewHeight, $transparent);

但问题是这些片段会进入被删除的图像大小调整部分。

我如何正确地修复透明度问题与我修改的代码?感谢您的阅读和帮助一个PHP新手。