WordPress template_include - how to hook it properly

You could use template_redirect as shown above, but that does require exit, and it does trample on everything else WordPress would normally do to find the current template. You may want to let that happen and then apply logic to the current template.

Using some of what is above...

add_action('template_include', 'mcd_set_template');function mcd_set_template() {    return locate_template('templatename.php');}

That is fairly simple, you can also pass an array to locate_template() to define a hierarchy. If you were to use 'template_redirect as shown above, you should still be using locate_template, and this is how.

add_action('template_redirect', 'mcd_set_template');function mcd_set_template() {      /**       * Order of templates in this array reflect their hierarchy.       * You'll want to have fallbacks like index.php in case yours is not found.       */      $templates = array('templatename.php', 'othertemplate.php', 'index.php');      /**       * The first param will be prefixed to '_template' to create a filter       * The second will be passed to locate_template and loaded.       */      include( get_query_template('mcd', $templates) );      exit;}

Finally, the best way would be to filter specific types instead of the whole hierarchy. For example you could filter 'category_template' or 'page_template'. That would be more specific, it would avoid messing with the whole template hierarchy if you don't want to - and it lets WordPress do more of the heavy lifting

For example:

add_filter('category_template', 'filter_category_template');function filter_category_template($template){    /* Get current category */    $category = get_queried_object();    /* Create hierarchical list of desired templates */    $templates = array (      'category.php',      'custom-category-template.php',       'category-{$category->slug}.php',      'category-{$category->term_id}.php',       'index.php'    );     return locate_template($templates);}

You can of course create that array of hierarchical templates any time you use locate_template(). Using this method, its easy to see how easily you could create all sorts of very detailed and specific hierarchies either as part of or separate from the native Template Hierarchy.

Have you tried using an add_action instead? For example, you might want to try something like the following in your plugin:

add_action('template_redirect', 'mcd_set_template');//Redirect to a preferred template.function mcd_set_template() {    $template_path = TEMPLATEPATH . '/' . "templatename.php";    if(file_exists($template_path)){        include($template_path);        exit;    }}

Here is a helpful reference: http://www.mihaivalentin.com/wordpress-tutorial-load-the-template-you-want-with-template_redirect/