<?php
add_filter( 'the_content', 'wds_enable_dev_mode' );
function wds_enable_dev_mode() {

    // Only alter the content on the donate page, and only if the devmode queryvar is not set to true
    if ( is_page('donate') && ( !isset($_GET['devmode']) || 'true' != $_GET['devmode'] )
        return '<h2>System Maintenance</h2><p>This page is currently undergoing maintenance. Sorry for the inconvenience. Please check back in an hour.</p>';

    // Otherwise, return content as normal
    return $content;
}}