get_meta_tags适用于URL，但不适用于本地文件

I'm trying to create a script that will read the meta tags from all other files in a directory and display information on each. When I try to do this referring to the local file name, it doesn't work. But for some reason, when I reference the complete URL, it works, but is too slow to be useful.

When I turn on error reporting, I get this error each time through the loop:

Undefined index: description in /path/to/my/script/index.php on line 83

Anybody know why remote files would work and local ones would not?

Here's the code that fails:

$dir   = '.';
$files = scandir($dir);
foreach ($files as &$value) {
   $tags = get_meta_tags($value);
   echo $tags['description'] . "<br>";
   }

But for some reason, this code works:

$dir   = '.';
$files = scandir($dir);
foreach ($files as &$value) {
   $tags = get_meta_tags('http://mydomain.com/path/' . $value);
   echo $tags['description'] . "<br>";
   }