Creates a submenu page for tools that lists out all custom page templates and which pages are using them. #wordpress

custom_page_templates.php

<?php
// Debug Custom Page Templates
function custom_page_templates_page() {
    $templates = wp_get_theme()->get_page_templates();
    $done = array();
    $row = '<tr><td><a href="%s">%s</a></td><td width="60"><em>%s</em></td><td width="20"><a href="%s">edit</a></td></tr>';
    $not_found = '<tr><td colspan="3"><em>No pages found using this template.</em></td></tr>';
    echo '<div class="wrap"><div id="icon-tools" class="icon32"></div>';
        echo '<h2>Custom Page Templates</h2>';
    foreach ( $templates as $template_name => $template_filename ):
        echo "<h3>$template_name ($template_filename)</h3>";
        echo '<table width="400">';
        $pages = get_pages(array(
            'meta_key'     => '_wp_page_template',
            'meta_value'   => $template_name,
            'hierarchical' => 0,
            'post_type'    => 'page'
        ));
        if(!$pages) echo $not_found;
        foreach($pages as $page){
            echo sprintf($row, get_permalink($page->ID), $page->post_title, $page->post_status, get_edit_post_link($page->ID));
        }
        $done[] = $template_name;
        echo '</table>';
    endforeach;
    $pages = glob(TEMPLATEPATH.'/page-*');
    foreach($pages as $p):
        $file = str_replace(trailingslashit(TEMPLATEPATH), '', $p);
    if(in_array($file, $done)) continue;
        preg_match('/page-([^.]+).php/', $file, $matches);
        $found = get_posts(array('name' => $matches[1], 'post_type' => 'page'));
        echo '<h3>'.$file.'</h3>';
        echo '<table width="400">';
        if($found):
            $page = $found[0];
            echo sprintf($row, get_permalink($page->ID), $page->post_title, $page->post_status, get_edit_post_link($page->ID));
        else:
            echo $not_found;
        endif;
        echo '</table>';
    endforeach;
    echo '</div>';
}
function register_tools_custom_templates_page() {
    add_management_page( 'Template Debugging', 'Custom Page Templates', 'manage_options', 'custom-page-templates', 'custom_page_templates_page' );
}
add_action('admin_menu', 'register_tools_custom_templates_page');