问题上传PNG图像是否透明？

I'm having some issues uploading PNG images with transparency.

I upload, crop and insert the images in the DB and when I try to show them in the website they come up black. It's only happening with PNG images, for JPG, JPEG and GIF it's working perfect.

Can somebody help me with that?

Thanks in advance!

Example

This is my code:

public function create_square_image($original_file, $destination_file=NULL, $square_size = 100){

    // get width and height of original image
    $imagedata = getimagesize($original_file);
    $original_width = $imagedata[0];    
    $original_height = $imagedata[1];

    if($original_width > $original_height){
        $new_height = $square_size;
        $new_width = $new_height*($original_width/$original_height);
    }
    if($original_height > $original_width){
        $new_width = $square_size;
        $new_height = $new_width*($original_height/$original_width);
    }
    if($original_height == $original_width){
        $new_width = $square_size;
        $new_height = $square_size;
    }

    $new_width = round($new_width);
    $new_height = round($new_height);

    // load the image
    if(substr_count(strtolower($original_file), ".jpg") or substr_count(strtolower($original_file), ".jpeg")){
        $original_image = imagecreatefromjpeg($original_file);
    }
    if(substr_count(strtolower($original_file), ".gif")){
        $original_image = imagecreatefromgif($original_file);
    }
    if(substr_count(strtolower($original_file), ".png")){


                    $original_image = imagecreatefrompng($original_file);       
    }

    $smaller_image = imagecreatetruecolor($new_width, $new_height);
    $square_image = imagecreatetruecolor($square_size, $square_size);

    imagecopyresampled($smaller_image, $original_image, 0, 0, 0, 0, $new_width, $new_height, $original_width, $original_height);

    if($new_width>$new_height){
        $difference = $new_width-$new_height;
        $half_difference =  round($difference/2);
        imagecopyresampled($square_image, $smaller_image, 0-$half_difference+1, 0, 0, 0, $square_size+$difference, $square_size, $new_width, $new_height);
    }
    if($new_height>$new_width){
        $difference = $new_height-$new_width;
        $half_difference =  round($difference/2);
        imagecopyresampled($square_image, $smaller_image, 0, 0-$half_difference+1, 0, 0, $square_size, $square_size+$difference, $new_width, $new_height);
    }
    if($new_height == $new_width){
        imagecopyresampled($square_image, $smaller_image, 0, 0, 0, 0, $square_size, $square_size, $new_width, $new_height);
    }


    // if no destination file was given then display a png      
    if(!$destination_file){
        imagepng($square_image,NULL,9);
    }

    // save the smaller image FILE if destination file given
    if(substr_count(strtolower($destination_file), ".jpg")){
        imagejpeg($square_image,$destination_file,100);
    }
    if(substr_count(strtolower($destination_file), ".gif")){
        imagegif($square_image,$destination_file);
    }
    if(substr_count(strtolower($destination_file), ".png")){
        imagepng($square_image,$destination_file,9);
    }

    imagedestroy($original_image);
    imagedestroy($smaller_image);
    imagedestroy($square_image);

}

You need to use imagesavealpha() in order to preserve the alpha channel when saving.