例：直近の過去１ヶ月分の日付を生成する

<?php

for ($d = (new DateTime())->modify('-1 month'); $d <= new DateTime(); $d->modify('+1 day')) {
    echo $d->format('Y-m-d'),"\n";
}

実行結果

2014-05-13
2014-05-14
2014-05-15
2014-05-16
2014-05-17
2014-05-18
...
[中略]
...
2014-06-12
2014-06-13