Black background instead of transparent on PNG images when processing them - php

Question

I'm having a script that is detecting weather it's a png image or jpeg, and also "removing" whitespace around images.
But I get a black background on all .png images. Why is that?
//load the image
$logo = $json['Logotype'];
$image_type = getimagesize($logo);
if($image_type['mime']=='image/jpeg') {
$img_type = 'jpeg';
$img = imagecreatefromjpeg($logo);
} elseif($image_type['mime']=='image/png') {
$img_type = 'png';
$img = imagecreatefrompng($logo);
}
//find the size of the borders
$b_top = 0;
$b_btm = 0;
$b_lft = 0;
$b_rt = 0;
//top
for(; $b_top < imagesy($img); ++$b_top) {
for($x = 0; $x < imagesx($img); ++$x) {
if(imagecolorat($img, $x, $b_top) != 0xFFFFFF) {
break 2; //out of the 'top' loop
}
}
}
//bottom
for(; $b_btm < imagesy($img); ++$b_btm) {
for($x = 0; $x < imagesx($img); ++$x) {
if(imagecolorat($img, $x, imagesy($img) - $b_btm-1) != 0xFFFFFF) {
break 2; //out of the 'bottom' loop
}
}
}
//left
for(; $b_lft < imagesx($img); ++$b_lft) {
for($y = 0; $y < imagesy($img); ++$y) {
if(imagecolorat($img, $b_lft, $y) != 0xFFFFFF) {
break 2; //out of the 'left' loop
}
}
}
//right
for(; $b_rt < imagesx($img); ++$b_rt) {
for($y = 0; $y < imagesy($img); ++$y) {
if(imagecolorat($img, imagesx($img) - $b_rt-1, $y) != 0xFFFFFF) {
break 2; //out of the 'right' loop
}
}
}
//copy the contents, excluding the border
$newimg = imagecreatetruecolor(
imagesx($img)-($b_lft+$b_rt), imagesy($img)-($b_top+$b_btm));
switch ($img_type)
{
case "png":
// integer representation of the color black (rgb: 0,0,0)
$background = imagecolorallocate($newimg, 0, 0, 0);
// removing the black from the placeholder
imagecolortransparent($newimg, $background);
// turning off alpha blending (to ensure alpha channel information
// is preserved, rather than removed (blending with the rest of the
// image in the form of black))
imagealphablending($newimg, false);
// turning on alpha channel information saving (to ensure the full range
// of transparency is preserved)
imagesavealpha($newimg, true);
break;
case "gif":
// integer representation of the color black (rgb: 0,0,0)
$background = imagecolorallocate($newimg, 0, 0, 0);
// removing the black from the placeholder
imagecolortransparent($newimg, $background);
}
imagecopy($newimg, $img, 0, 0, $b_lft, $b_top, imagesx($newimg), imagesy($newimg));
//finally, output the image
header("Content-Type: image/" . $img_type . "");
imagejpeg($newimg);

Answer 1

This $background = imagecolorallocate($newimg, 0, 0, 0); is for black background
use $background = imagecolorallocatealpha($newimg, 255, 255, 255); instead