Get last date form given date

<?php

$periode = "2020-08-01";
$periodeStart = $periode." 00:00:00";
$periodeEnd = $periode." 23:59:59";
$year = date("Y");
$lastDate = (new DateTime("$periodeStart"))->modify("last day of")->format("Y-m-d");
$periodeEnd = "$lastDate 23:59:59";

echo "$periodeStart\n";
echo "$periodeEnd\n";