回答

<?php

/**
* 获取指定月份段内每一个月份
* @param Date $startmonth 开始日期
* @param Date $endmonth  结束日期
* @return Array
*/
function getMonthFromRange($startmonth, $endmonth) {
    $stimestamp = strtotime($startmonth);
    $etimestamp = strtotime($endmonth);
    // 计算日期段内有多少天
   // $days = ($etimestamp-$stimestamp)/86400+1;

    $y = date('Y', $stimestamp);

    $ys = date('Y', $etimestamp);

    $m = (int)date('m', $stimestamp);

    $ms = (int)date('m', $etimestamp);

    $chaY = $ys-$y;

    //月份相差多少

    $chaM = 12-$m + $ms;

    //相差一年就加12

    $yearmeth = $chaM + (($chaY-1) *12);

    //echo $yearmeth;
    // 保存每天日期
    $month = array();
    for ($i = 0; $i <= $yearmeth; $i++) {
        $month[] = date('Y-m', strtotime("+{$i}month",$stimestamp));
    }
    return $month;
}

$startdate = '2021-08';
$enddate = '2022-09';
// demo
$date = getMonthFromRange($startdate, $enddate);
print_r($date);