Can someone help with a php code for birthday validation so that student below 20 year's wouldn't be able to register

Question

PHP Code for Form validation

Answer 1

<input type = 'number' name ='d'>
<?php
$d=$_POST['d'];
if($d<2000)
echo 'above 20';
?>

Answer 2

https://code.sololearn.com/WHriFLBaxSh5/?ref=app