Fix png transparency problem - php

Question

I use this class to upload images on my server. The problem is that when I upload transparent png images, the transparent parts get black. Is there a way to keep transparency in png images?
class SimpleImage {
var $image;
var $image_type;
function load($filename) {
$image_info = getimagesize($filename);
$this->image_type = $image_info[2];
if( $this->image_type == IMAGETYPE_JPEG ) {
$this->image = imagecreatefromjpeg($filename);
} elseif( $this->image_type == IMAGETYPE_GIF ) {
$this->image = imagecreatefromgif($filename);
} elseif( $this->image_type == IMAGETYPE_PNG ) {
$this->image = imagecreatefrompng($filename);
}
}
function save($filename, $image_type=IMAGETYPE_JPEG, $compression=100, $permissions=null) {
if( $image_type == IMAGETYPE_JPEG ) {
imagejpeg($this->image,$filename,$compression);
} elseif( $image_type == IMAGETYPE_GIF ) {
imagegif($this->image,$filename);
} elseif( $image_type == IMAGETYPE_PNG ) {
imagepng($this->image,$filename);
}
if( $permissions != null) {
chmod($filename,$permissions);
}
}
function output($image_type=IMAGETYPE_JPEG) {
if( $image_type == IMAGETYPE_JPEG ) {
imagejpeg($this->image);
} elseif( $image_type == IMAGETYPE_GIF ) {
imagegif($this->image);
} elseif( $image_type == IMAGETYPE_PNG ) {
imagepng($this->image);
}
}
function getWidth() {
return imagesx($this->image);
}
function getHeight() {
return imagesy($this->image);
}
function resizeToHeight($height) {
$ratio = $height / $this->getHeight();
$width = $this->getWidth() * $ratio;
$this->resize($width,$height);
}
function resizeToWidth($width) {
$ratio = $width / $this->getWidth();
$height = $this->getheight() * $ratio;
$this->resize($width,$height);
}
function scale($scale) {
$width = $this->getWidth() * $scale/100;
$height = $this->getheight() * $scale/100;
$this->resize($width,$height);
}
function resize($width,$height) {
$new_image = imagecreatetruecolor($width, $height);
imagecopyresampled($new_image, $this->image, 0, 0, 0, 0, $width, $height, $this->getWidth(), $this->getHeight());
$this->image = $new_image;
}
}

Answer 1

I don't beleive that this class can handle transparent PNGs. The PHP documentation on imagecopyresampled explains this.
You'll probably need to extend the class to use imagecolorallocatealpha.
Read for reference: http://php.net/manual/en/function.imagecreatetruecolor.php

Answer 2

I used this one to ramain the transparency of a png or gif images.
imagecolortransparent($this->imageResized, imagecolorallocatealpha($this->imageResized, 0, 0, 0, 127));
imagealphablending($this->imageResized, false);
imagesavealpha($this->imageResized, true);

Answer 3

You may need to use
imagealphablending($this->image, false);
imagesavealpha($this->image, true);
before imagepng

Answer 4

Solution can be found here:
PNG Transparency Resize with SimpleImage.php Class
it modifies a bit the class in order to allow transparencies in case of PNG and GIF images.