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();
    $multi = array(
        'y' => 365,
        'm' => 30,
        'd' => 1
    );
    if ($invoice->first_total>0 && !($invoice->second_total>0)) {

        $first_period = null; 
        foreach ($invoice->getItems() as $item) {
            if ($item->first_price>0) {
                if ($item->first_period == Am_Period::MAX_SQL_DATE) {
                    $first_period = Am_Period::MAX_SQL_DATE;
                    break;
                }
                $p = new Am_Period($first_period ?: $item->first_period);
                $np = new Am_Period($item->first_period);
                $first_period =
                    $p->getCount() * $multi[$p->getUnit()] > $np->getCount() * $multi[$np->getUnit()] ?
                    strval($p) : strval($np);
            }
        }
        $invoice->first_period = $first_period;
    }
});