<pre>
<?php

$dates = array(
    array(1, 2),
    array(3, 4),
    array(5, 6),
    array(7, 8),
    array(9, 10),
    array(11, 12),
);

foreach($dates as &$date)
{
    $date[2] = 'a' . $date[0];
}

var_dump($dates);

for($j = 0; $j < 2; $j++)
{
    foreach($dates as $i => $date)
    {
        echo $i . ' : ' . $date[0] . ' - ' . $date[2] . PHP_EOL;
    }
    echo '</hr >';
}

?>
</pre>