使用PHP操作日期时间

Question

$arr = array("0" => array(
               "from" => '2011-12-22 06:30',
               "to" => '2011-12-22 09:00'
        ),
             "1" => array(
               "from" => '2011-12-22 10:30',
               "to" => '2011-12-22 11:00'
        ),
             "2" => array(
               "from" => '2011-12-22 12:30',
               "to" => '2011-12-22 13:00'
        )
    );


$datetime = new DateTime("06:00");

echo "<table>"; 


for($i=0; $i<20; $i++) { 
    echo "<tr>";
    $datetime->modify('+30 minutes');

    for ($j=0; $j<7; $j++){
        if($arr[0]['from'] == $datetime->format('Y-m-d H:i') ) {
                $class = ' class="red"';
            } else {
                $class = '';
            };
        echo "<td $class>" . $datetime->format('Y-m-d H:i') . "</td>";
        $datetime->modify('+1 day');  
    }

    echo "</tr>";
}

echo "</table>";

现在我只检查$ arr * [0] *。是否可以检查此阵列中的所有位置（当前0,1,2）？现在这只检查最后一个元素。我想检查数组中的所有数据。