WordPress – Template Dropdown not showing up within WordPress administrator page

You don't need to interact with the API or use a plugin.

First, duplicate post.php or page.php in your theme folder (under /wp-content/themes/themename/).

Rename the new file as templatename.php (where templatename is what you want to call your new template). To add your new template to the list of available templates, enter the following at the top of the new file:

<?php
/*
Template Name: Name of Template
*/
?>

You can modify this file (using PHP) to include other files or whatever you need.

Then create a new page in your WordPress blog, and in the page editing screen you'll see a Template dropdown in the Attributes widget to the right. Select your new template and publish the page.

Your new page will use the PHP code defined in templatename.php

Source: Creating Custom Page Templates for Global Use

The WordPress global variable $pagename should be available for you. I have just tried with the same setup you specified.

$pagename is defined in the file wp-includes/theme.php, inside the function get_page_template(), which is of course is called before your page theme files are parsed, so it is available at any point inside your templates for pages.

• Although it doesn't appear to be documented, the $pagename var is only set if you use permalinks. I guess this is because if you don't use them, WordPress doesn't need the page slug, so it doesn't set it up.

• $pagename is not set if you use the page as a static front page.

• This is the code inside /wp-includes/theme.php, which uses the solution you pointed out when $pagename can't be set:

--

if ( !$pagename && $id > 0 ) {
  // If a static page is set as the front page, $pagename will not be set. Retrieve it from the queried object
  $post = $wp_query->get_queried_object();
  $pagename = $post->post_name;
}