
<?php

/**
 * 今日の日時なら時間と分だけに刈る
 *
 * 2012-01-01 19:14:21 -> 19:14
 *
 * @param array &$rows
 */
function trimDatetime(&$rows) {
    foreach ($rows as &$row) {
        if (substr($row['created_at'], 0, 10) === date('Y-m-d')) {
            $row['created_at'] = substr($row['created_at'], 11, 5);
        }
    }
}

$rows = array(
    array(
        'id' => 1,
        'created_at' => date('Y-m-d 19:14:21')
    ),
    array(
        'id' => 1,
        'created_at' => '2012-01-01 19:14:21'
    ),
);

echo '[before]' . PHP_EOL;
echo $rows[0]['created_at'] . PHP_EOL;
echo $rows[1]['created_at'] . PHP_EOL;

trimDateTime($rows);

echo '[after]' . PHP_EOL;
echo $rows[0]['created_at'] . PHP_EOL;
echo $rows[1]['created_at'] . PHP_EOL;

結果↓

[before]
2012-09-26 19:14:21
2012-01-01 19:14:21
[after]
19:14
2012-01-01 19:14:21