How to find out if a (remote) URL is an image?

Question

This was asked before, but nothing works for me:

• getimagesize is not recommended to use for this purpose by the PHP manual.
• get_headers as in this example don't find any headers.
• exif_imagetype refuses to check a remote URL.
• I have also tried one 'curl' example (not sure which), and it didn't work for me either.

What I want to do: If the URL is pointing at an image, present it as <img>. Otherwise <a>.

Edit1: Based on the solution linked above, I extract the extension. It works, but very unreliable:

if(filter_var($url, FILTER_VALIDATE_URL))
{
    $extension = strtolower(strtok(pathinfo($url, PATHINFO_EXTENSION),'?'));

    $valid_image_type=array();
    $valid_image_type['png']='';
    $valid_image_type['jpg']='';
    $valid_image_type['jpeg']='';
    $valid_image_type['gif']='';
    $valid_image_type['tif']='';
    $valid_image_type['tiff']='';
    $valid_image_type['svg']='';
    $valid_image_type['ico']='';
    $valid_image_type['icon']='';
    $valid_image_type['x-icon']='';

    if(isset($valid_image_type[$extension])){
        $base = "<img src=\"$url\" >";
    }
    else
    {
        $base = "<a href=\"$url\">...</a>";
    }

}
else
{
    // Plain text
}

switch is a multiple OR, so the code above could be simplified. The strtok removes the arguments following the extension:

switch (strtolower(strtok(pathinfo($url, PATHINFO_EXTENSION),'?'))) {
      case 'png':
      case 'jpg':
      case 'jpeg':
      case 'gif':
      case 'tif':
      case 'tiff':
      case 'svg':
      case 'ico':
      case 'icon':
      case 'x-icon':
        // URL is an image
      break;
      default:
        // URL is not an image
    }