projects.php

<?php

$id = $modx->resource->get('id');
$q = $modx->newQuery('modResource');
$q->where(array('template'=>3, 'published'=>1, 'deleted:!='=>1));
$q->sortby('menuindex', 'ASC');

$q->select('id,uri');
if ($q->prepare() && $q->stmt->execute()) {
  $projects = $q->stmt->fetchAll(2);
}
foreach($projects as $k => $p){
    if($p['id'] == $id){
        $next_uri = $projects[$k+1]['uri'];
        $prev_uri = $projects[$k-1]['uri'];
    }
    if ($p === reset($projects))
       $first = $p;

    if ($p === end($projects))
        $last = $p;
}

if(!isset($next_uri) || empty($next_uri)){
    $next_uri = $first['uri'];
}

if(!isset($prev_uri) || empty($prev_uri)){
    $prev_uri = $last['uri'];;
}

return $modx->setPlaceholders(array('next_project'=>$next_uri, 'prev_project'=>$prev_uri));