Code:

function createthumb($name,$filename,$new_w,$new_h) { 
  $system=explode('.',$name);
  if (preg_match('/jpg|jpeg/',$system[1])) { 
    $src_img=imagecreatefromjpeg($name);
  }
  if (preg_match('/png/',$system[1])) {
    $src_img=imagecreatefrompng($name);
  }

  $old_x=imageSX($src_img);
  $old_y=imageSY($src_img);
  if ($old_x > $old_y) {
    $thumb_w=$new_w;
    $thumb_h=$old_y*($new_h/$old_x);
  }
  if ($old_x < $old_y) {
    $thumb_w=$old_x*($new_w/$old_y);
    $thumb_h=$new_h;
  }
  if ($old_x == $old_y) {
    $thumb_w=$new_w;
    $thumb_h=$new_h;
  }

  $dst_img=ImageCreateTrueColor($thumb_w,$thumb_h);
  imagecopyresampled($dst_img,$src_img,0,0,0,0,$thumb_w,$thumb_h,$old_x,$old_y);

  if (preg_match("/png/",$system[1])) {
    imagepng($dst_img,$filename);
  }
  else {
    imagejpeg($dst_img,$filename);
  }

  imagedestroy($dst_img);
  imagedestroy($src_img);
}