<?php
$itemsToProcess = array(1,2,3);

foreach ($itemsToProcess as &$item) {
    if ($item == 2) {
        // processing of item 2 produces 2 new items that should be processed:
        $itemsToProcess[] = 7;
        $itemsToProcess[] = 8;
        continue;
    }
    echo $item;
}