Validate a date in PHP

You can use the function PHP called strtotime

  

Here the link of the documentation > Strtotime

Here is an example of this method in execution ...

<?php
    date_default_timezone_set("America/Mexico_City");
    $end = '2017-12-29';
    $fecha_actual = strtotime(date("Y-m-d H:i:s"));
    $fecha_nacimiento = strtotime($end);
        
        if($fecha_actual > $fecha_nacimiento){
            echo "fecha correcta";
        }else{
            echo "fecha incorrecta";
        }                   
?>

With date_default_timezone_set we set the time zone (I put it since several times PHP returns me incorrect hours even if my php.ini has correctly set the zone)

In the variable $end we establish the date of birth, which may well be received from a form with $end = $_POST['idElementoDondeSeIngresaLaFechaDeNacimiento'];

In the variable $fecha_actual we collect the date with format year-month (number) -dia (number) hour (with 24 hour format) -minutes-seconds

And the if simply compares both variables, I hope it serves you, greetings.

If you are more specific about how you received the date, it could be more explicit but I leave this:

$fecha_recibida = $_POST['fecha']; //Tiene que estar en el formato 2017-11-29
$fecha_enviada = new DateTime(date('Y-m-d',$fecha_recibida));
$fecha_actual = new DateTime(date('Y-m-d'));

if ($fecha_actual > $fecha_enviada) {
    # code
}