How to resize a gif in PHP and maintain background?

Question

I found this code here at the stack (Creating a thumbnail from an uploaded image), and it works fine for images (png, jpeg, jpg) but when i try with the same built-in function it just doesn't work with gif's (http://php.net/manual/en/function.imagecreatefromgif.php). Furthermore whenever you upload an image and use the function to resize the image the resized image's background is black.

So my questions are

1. what do i do to prevent the background from turning black?

2. how do i extend the function so it also covers gif's?

   function createThumbnail($type,$image_name,$new_width,$new_height,$uploadDir,$moveToDir){
   $path = $uploadDir . $image_name;
   $mime = getimagesize($path);
   if($mime['mime']=='image/png') { 
       $src_img = imagecreatefrompng($path);
   }
   if($mime['mime']=='image/gif') { 
       $src_img = imagecreatefromgif($path);
   }
   if($mime['mime']=='image/jpg' || $mime['mime']=='image/jpeg' || $mime['mime']=='image/pjpeg') {
       $src_img = imagecreatefromjpeg($path);
   }   
   $old_x      =   imageSX($src_img);
   $old_y      =   imageSY($src_img);
   $thumb_w    =   $new_width;
   $thumb_h    =   $new_height;
   $dst_img    =   ImageCreateTrueColor($thumb_w,$thumb_h);
   imagecopyresampled($dst_img,$src_img,0,0,0,0,$thumb_w,$thumb_h,$old_x,$old_y); 
   // New save location
   $image_name_new = explode(".", $image_name);
   if ($type == 1){
       $image_name = $image_name_new[0] . "_thumb." . $image_name_new[1];
   } else if ($type == 2){
       $image_name = $image_name_new[0] . "_image." . $image_name_new[1];
   }
   $new_thumb_loc = $moveToDir . $image_name;
   if($mime['mime']=='image/png') {
       $result = imagepng($dst_img,$new_thumb_loc,8);
   }
   if($mime['mime']=='image/gif') {
       $result = imagegif($dst_img,$new_thumb_loc,8);
   }
   if($mime['mime']=='image/jpg' || $mime['mime']=='image/jpeg' || $mime['mime']=='image/pjpeg') {
       $result = imagejpeg($dst_img,$new_thumb_loc,80);
   }
   imagedestroy($dst_img); 
   imagedestroy($src_img);return $result;}