Your second function is wrong. get_post_type($post)
excepts the current post object or post ID or any post object or ID given to it. You are using the arguments for get_post_types( $args, $output, $operator )
for get_post_type
, which fails the function which in turn gives you the error in your foreach
loop.
To rectify your function, use something like this:
add_filter('single_template', 'my_single_template_folders_terms');
function my_single_template_folders_terms($template) {
global $post;
$post_type = get_post_type($post);
if ( file_exists(TEMPLATEPATH . "/{$post_type}/single.php") )
return TEMPLATEPATH . "/{$post_type}/single.php";
return $template;
}