php 一ヶ月前の日付

<?php
$date = new DateTime();
$prev_date = $date->modify('-1 month')->format('Y-n');//format('Y-m');ならゼロうめ表示
$prev_date =substr($prev_date, 5, 2);
//$prev_date = ltrim($prev_date, '0');
echo $prev_date;  // 7とか