404.php

<?php
    /**
     * The template for displaying 404 pages (not found).
     *
     * @link https://codex.wordpress.org/Creating_an_Error_404_Page
     *
     * @package RedRock
     */

    get_header();
?>

<div id="primary" class="content-area">
    <main id="main" class="site-main" role="main">
        <section class="error-404 not-found">
            <header class="page-header">
                <h1 class="page-title">That page cannot be found.</h1>
            </header>
            <div class="page-content">
                <p>The page you want has moved or does not exist. You might find what you want if you do a search.</p>
<?php
                if (isset($_GET["id"])) {
                    $searchPrefill = $_GET["id"];
                }
                else {
                    $searchPrefill = basename(strtok($_SERVER["REQUEST_URI"], "?"));
                }
                $searchPrefill = preg_replace('/^\d+-/', '', $searchPrefill);
                $searchPrefill = preg_replace('/^article-/', '', $searchPrefill);
                $searchPrefill = preg_replace('/[^a-zA-Z0-9]+/', ' ', $searchPrefill);
                $searchPrefill = trim($searchPrefill);
                include(locate_template('searchform.php', false, false));
?>
            </div>
        </section>
    </main>
</div>

<?php get_footer(); ?>