Choose template depending on URL

Question

I have the following basic page setup:

• xxx.net/ -> static homepage
• xxx.net/blog/ -> blog page with loop showing all posts
• xxx.net/gallery/ -> gallery page with loop only showing posts that are categorized as "gallery"
• xxx.net/extra/ -> another page with loop only showing posts that are categorized as "extra"

The two special categories "gallery" and "extra" are used to filter certain posts and display them in a different way:

On /gallery, only posts of category "gallery" are shown (actually there custom excerpts rather than their standard content); the equivalent holds true for "extra".

So far that's all clear to me, however depending on whether a post is shown as part of the blog loop or the other loops, different post templates are used: if a post is called via xxx.net/gallery/post-name, the gallery template is used, if called via xxx.net/blog/post-name, the standard template is used.

How can I do this? I somehow think the answer is simple but I just don't see it right now! Would it make sense to use a new, custom taxonomy rather than categories to distinguish the "special" posts? The main question regarding permalinks remains the same however.

Addition:

I guess I could create a single.php with the following switcher:

if ( URL INCLUDES /GALLERY/ ) {include (TEMPLATEPATH . '/single-gallery.php');}
else if ( URL INCLUDES /EXTRA/ ) { include (TEMPLATEPATH . '/single-extra.php');}
else { include (TEMPLATEPATH . '/single-blog.php'); }

However I think a rewrite rule or similar would be the better approach. What is your take on the topic?

Second Addition:

To further explain why I would like this functionality: Think of a trip report. It contains some text (the actual trip report) plus a collection of photos (the trip gallery). Instead of seperating the two things by creating two different posts, I would like to create only one post with the text as content and the photos as attachments.

Then I would also like to allow the visitor to

a) read the post as a standard blog entry with standard output without the photos by calling .../blog/post-name

b) view the post as a photo album (e.g. thumbnail page) by calling .../gallery/post-name

Answer 1

One post cannot (and should not) have two single post URLs. All you can change is the archive view, the list of posts.

If /gallery/ and /extra/ are post of the post type page, use a custom page template for those.

Update

Create two endpoints gallery and extra for the root (EP_ROOT) (another example). The callback function for those endpoints the has two options:

1. If the endpoint variable value is TRUE (/gallery/ was called without a post name), render an archive listing.
2. If there is a string value, use get_page_by_path() and fetch the post object. Render it.

Example for get_page_by_path():

$post = get_page_by_path( 
    get_query_var( 'gallery' ), 
    OBJECT, 
    'post' 
);