date.php

<?php

    function validateFormat(string $date): DateTime
    {
        $formats = ['dmY', 'd/m/Y', 'Ymd', 'Y-m-d'];

        foreach ($formats as $format) {

            $date_format = DateTime::createFromFormat($format, $date);

            if ($date_format && $date_format->format($format) === $date) {
                return $date_format;
            }
        }
    }

    $data = '29032021';
    echo $this->validateFormat($data)->format('Y-m-d'); //2021-03-29