No connection could be made error in phpThumb()

I'm using phpThunmb to display images from external link. for some images, such type of error occurred as below.

i tried to get the header information of that image as below..

 $headers = get_headers($external_image_url);

but the result was empty. how to solve this error? if this image can not be accessed then how to display an alternate "image imageNotFound.jpg" in it's place .

try this code to check if the given image exists or not.

if(file_exists($external_image_url)) 
{
  $image_link = $external_image_url; //if that image exists
 }
 else
    {
      $image_link = "imageNotFount.jpg"; //if that image does not exists
    }