<?php

$string = "09/07/2021";
$padrao = "/^[0-9]{2}\/[0-9]{2}\/[0-9]{4}$/";

if(preg_match($padrao, $string)):
    echo "Válido";
    echo "<hr>";
    echo $string;
else:
    echo "Inválido";
    echo "<hr>";
endif;