Moodle "next" and "previous" activity code

get_activity_url.php

<?php

// you would probably use this within your theme or block - anywhere inside a course context.

// a moodle function for returning the url adjacent to the current page in a course
// tested in Moodle 3 and higher.
// sourced from within https://moodle.org/plugins/block_navbuttons then refactored a bit to my needs

// lets say you are on http://mymoodle.com/mod/page/view.php?id=302
// and you want to know what the activity immediately after this one is. You'd ask
// $url = get_activity_url(course_get_url($course, $cm->sectionnum),"next")
// $url would be the location of the next visible activity in the sequence, or the course/section if there wasn't a next activity.

// licence: GPL3, as is Moodle

function get_activity_url($default, $direction = "next") {
global $PAGE, $CFG, $COURSE, $DB;

    require_once($CFG->libdir . '/modinfolib.php');

    $cmid = $PAGE->cm->id;
    $modinfo = get_fast_modinfo($COURSE);
    $context = context_course::instance($COURSE->id);
    $sections = $DB->get_records('course_sections', array('course' => $COURSE->id), 'section', 'section,visible,summary');

    $next = null;
    $prev = null;
    $firstcourse = null;
    $firstsection = null;
    $lastcourse = null;
    $lastsection = null;

    $sectionnum = -1;
    $thissection = null;
    $firstthissection = null;
    $flag = false;
    $sectionflag = false;
    $previousmod = null;

    foreach ($modinfo->cms as $mod) {
        if ($mod->modname == 'label') {
            continue;
        }
        $format = course_get_format($COURSE);
        if (method_exists($format, 'get_last_section_number')) {
            $numsections = $format->get_last_section_number();
        } else {
            $opts = course_get_format($COURSE)->get_format_options();
            $numsections = isset($opts['numsections']) ? $opts['numsections'] : 0;
        }
        if ($numsections && $mod->sectionnum > $numsections) {
            break;
        }
        if (!$mod->uservisible) {
            continue;
        }
        if ($mod->sectionnum > 0 && $sectionnum != $mod->sectionnum) {
            $thissection = $sections[$mod->sectionnum];
            if ($thissection->visible || !$COURSE->hiddensections ||
                has_capability('moodle/course:viewhiddensections', $context)
            ) {
                $sectionnum = $mod->sectionnum;
                $firstthissection = false;
                if ($sectionflag) {
                    if ($flag) { // Flag means selected mod was the last in the section.
                        $lastsection = 'none';
                    } else {
                        $lastsection = $previousmod;
                    }
                    $sectionflag = false;
                }
            } else {
                continue;
            }
        }
        $thismod = (object)[
            'link' => new moodle_url('/mod/'.$mod->modname.'/view.php', array('id' => $mod->id)),
            'name' => strip_tags(format_string($mod->name, true))
        ];
        if ($flag) { // Current mod is the 'next' mod.
            $next = $thismod;
            $flag = false;
        }
        if ($cmid == $mod->id) {
            $flag = true;
            $sectionflag = true;
            $prev = $previousmod;
            $firstsection = $firstthissection;
        }
        if (!$firstthissection) {
            $firstthissection = $thismod;
        }
        if (!$firstcourse) {
            $firstcourse = $thismod;
        }
        $previousmod = $thismod;
    }

    $result = $default;

    switch ($direction) {
        case "next":
            if ($next) {
                $result = $next->link;
            }
            break;

        case "prev":
            if ($prev) {
                $result = $prev->link;
            }
            break;

        case "first":
            if ($firstcourse) {
                $result = $firstcourse->link;
            }
            break;

        case "last":
            if ($lastcourse) {
                $resut = $lastcourse->link;
            }

        case "firstsection":
            if ($firstsection) {
                $result = $firstsection->link;
            }
            break;

        case "lastsection":
            if ($lastsection) {
                $result = $lastsection->link;
            }
            break;

        case "home":
            $result = new moodle_url('/course/view.php', array('id' => $COURSE->id));
            break;
    }

    return $result;

}