By default, you will get black background if you resize a transparent image. To fix it, you need set alpha channel imagecolorallocatealpha to 127.

imagecolorallocatealpha() behaves identically to imagecolorallocate() with the addition of the transparency parameter alpha which may have a value between 0 and 127. 0 indicates completely opaque while 127 indicates completely transparent.

main page:

<?
include_once("resizeimage.php");
$img = "image.png"; // File image location
$newfilename = "thumb_image.png"; // New file name for thumb
$w = 100;
$h = 100;
 
$thumbnail = resize($img, $w, $h, $newfilename);
 
echo "<img src='".$thumbnail."'>";?&gt;

resizeimage.php:

<?
function resize($img, $w, $h, $newfilename) {
 
 //Check if GD extension is loaded
 if (!extension_loaded('gd') && !extension_loaded('gd2')) {
  trigger_error("GD is not loaded", E_USER_WARNING);
  return false;
 }
 
 //Get Image size info
 $imgInfo = getimagesize($img);
 switch ($imgInfo[2]) {
  case 1: $im = imagecreatefromgif($img); break;
  case 2: $im = imagecreatefromjpeg($img);  break;
  case 3: $im = imagecreatefrompng($img); break;
  default:  trigger_error('Unsupported filetype!', E_USER_WARNING);  break;
 }
 
 //If image dimension is smaller, do not resize
 if ($imgInfo[0] <= $w && $imgInfo[1] <= $h) {
  $nHeight = $imgInfo[1];
  $nWidth = $imgInfo[0];
 }else{
                //yeah, resize it, but keep it proportional
  if ($w/$imgInfo[0] > $h/$imgInfo[1]) {
   $nWidth = $w;
   $nHeight = $imgInfo[1]*($w/$imgInfo[0]);
  }else{
   $nWidth = $imgInfo[0]*($h/$imgInfo[1]);
   $nHeight = $h;
  }
 }
 $nWidth = round($nWidth);
 $nHeight = round($nHeight);
 
 $newImg = imagecreatetruecolor($nWidth, $nHeight);
 
 /* Check if this image is PNG or GIF, then set if Transparent*/  
 if(($imgInfo[2] == 1) OR ($imgInfo[2]==3)){
  imagealphablending($newImg, false);
  imagesavealpha($newImg,true);
  $transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
  imagefilledrectangle($newImg, 0, 0, $nWidth, $nHeight, $transparent);
 }
 imagecopyresampled($newImg, $im, 0, 0, 0, 0, $nWidth, $nHeight, $imgInfo[0], $imgInfo[1]);
 
 //Generate the file, and rename it to $newfilename
 switch ($imgInfo[2]) {
  case 1: imagegif($newImg,$newfilename); break;
  case 2: imagejpeg($newImg,$newfilename);  break;
  case 3: imagepng($newImg,$newfilename); break;
  default:  trigger_error('Failed resize image!', E_USER_WARNING);  break;
 }
   
   return $newfilename;
}
?>

a more simple version:

<?
$newImg = imagecreatetruecolor($nWidth, $nHeight);
imagealphablending($newImg, false);
imagesavealpha($newImg,true);
$transparent = imagecolorallocatealpha($newImg, 255, 255, 255, 127);
imagefilledrectangle($newImg, 0, 0, $nWidth, $nHeight, $transparent);
imagecopyresampled($newImg, $im, 0, 0, 0, 0, $nWidth, $nHeight, $imgInfo[0], $imgInfo[1]);
?>