Passing variables between category.php to single.php?

Question

What I can't figure out is a method to have the current category id passed to the header.php when a post is clicked.

So let's say I'm on category 2 and I click on a post that belongs to multiple categories. I want the single.php to maintain the appearance of being on category 2. In order to do this I would need a variable to tell the header which category it came from

Answer 1

Simply name your template category-X.php, where X is the specific category name or id you want a custom template for, and place it in a /single folder in your main theme directory. Now any time a single post is called and it matches an existing template in that folder, it will use that to display the post instead of the regular single.php. If no match is found then single.php is used.

in functions.php:

define(SINGLE_PATH, TEMPLATEPATH . '/single');
add_filter('single_template', 'force_cat2single_template');  

function force_cat2single_template($single) {
global $wp_query, $post;
foreach((array)get_the_category() as $cat) :

        if(file_exists(SINGLE_PATH . '/single-cat-' . $cat->slug . '.php'))
            return SINGLE_PATH . '/single-cat-' . $cat->slug . '.php';

        elseif(file_exists(SINGLE_PATH . '/single-cat-' . $cat->term_id . '.php'))
            return SINGLE_PATH . '/single-cat-' . $cat->term_id . '.php';
        endforeach;

        if(file_exists(SINGLE_PATH . '/single.php'))
            return SINGLE_PATH . '/single.php';
        elseif(file_exists(SINGLE_PATH . '/default.php'))
            return SINGLE_PATH . '/default.php';
    return $single;
}