This code fragment is intended to be added into a XOOPS installation's mainfile.php. If the invoking URL does not match the scheme declared in XOOPS_URL (i.e. "https",) it will force a 301 redirect to the matching URL with the declared scheme.

scheme-301.php

<?php
    // add these lines to mainfile.php directly below the line "define('XOOPS_URL', ...);"
    $checkScheme = function () {
        $s = $_SERVER;
        $parsed = parse_url(XOOPS_URL);
        $targetScheme = $parsed['scheme'];
        $currentScheme = (!empty($s['HTTPS']) && $s['HTTPS'] !== 'off') ? 'https' : 'http';
        if (0 !== strcmp($targetScheme, $currentScheme)) {
            $host = $parsed['host'];
            $port = isset($parsed['port']) ? ':' . $parsed['port'] : '';
            $fullUrl = $targetScheme . '://' . $host . $port . $s['REQUEST_URI'];
            header('Location: ' . $fullUrl, true, 301);
            exit();
        }
    };
    $checkScheme();