Combine terms in case of multiple items within invoice with custom logic

site.php

<?php
Am_Di::getInstance()->hook->add(Am_Event::INVOICE_TERMS, function(Am_Event $e) {

    $invoice = $e->getInvoice();
    if ($invoice->first_total>0 && !($invoice->second_total>0)) {
        $n = new DateTime('00:00');
        $f = clone $n;

        foreach ($invoice->getItems() as $item) {
            if ($item->first_total > 0) {
                $n->add(new DateInterval("P" . strtoupper($item->first_period)));
            }
        }

        $i = $f->diff($n);
        $period = array();
        foreach (array(
            'y' => array('Year', 'Years'),
            'm' => array('Month', 'Months'),
            'd' => array('Day', 'Days')) as $token => $label) {

            if (($_ = $i->$token) > 0) {
               $period[] = sprintf('%d %s', $_, $_ > 1 ? $label[1] : $label[0]);
            }
        }
        $terms = sprintf('%s for %s',
            Am_Currency::render($invoice->first_total, $invoice->currency),
            implode(' and ', $period));
        $e->setReturn($terms);
    }

});