Map of email threads PHP 5.5

<?php

/**
 * Threads MAP
 */

$threads = array(
    5  => array(5,6),
    10 => array(10,11),
);

/**
 * Emails list
 */

$email5 = new stdClass;
$email5->subject = 'Cloud Storage Dump';
$email5->to = 'jondoe@domain.com';
$email5->msgno = 5;

$email6 = new stdClass;
$email6->subject = 'Cloud Storage Dump 6';
$email6->to = 'jondoe@domain6.com';
$email6->msgno = 6;

$email10 = new stdClass;
$email10->subject = 'Cloud Storage Dump 10';
$email10->to = 'jondoe@domain10.com';
$email10->msgno = 10;

$email11 = new stdClass;
$email11->subject = 'Cloud Storage Dump 11';
$email11->to = 'jondoe@domain11.com';
$email11->msgno = 11;

$emails = array($email5, $email6, $email10, $email11);



/**
 * Create map of key-threads
 */

function genetateKeyMap($emails)
{
    foreach($emails as $k => $email)
    {
        yield $email->msgno => $k;
    }
};
$keys = iterator_to_array(genetateKeyMap($emails));


/**
 * Assign email details to thread map
 */

function updateThreads($emails, $threads, $keys)
{
    foreach($threads as $thread)
    {
        $array = array();
        foreach($thread as $msgno)
        {
            $array[] = $emails[$keys[$msgno]];
        }
        yield $array;
    }
};
$threads = iterator_to_array(updateThreads($emails, $threads, $keys));


echo '<br><hr><br /><pre>'. print_r($threads, true). '</pre>';