How to easily hide an echo after submit in a PHP form?

I'm working with cookies and I need to show a message saying that 'x' cookie is not set until I introduce a value in a form, then after I click submit I need to show a message saying that 'x' cookie is set, but I need to 'hide' the previous message.

<?php
$cookie_name = NULL;
$cookie_value = NULL;
$cookie_name = "user";
$cookie_value = "Name Surname";

setcookie($cookie_name, $cookie_value, time()+86400 );

echo "Cookie named '" . $cookie_name . "' is not set!";

if($_SERVER["REQUEST_METHOD"]=="POST"){
  if(!isset($_COOKIE[$cookie_name])) {
     echo "Cookie named '" . $cookie_name . "' is not set!";
} else {
     echo "Cookie '" . $cookie_name . "' is set!<br>";
     echo "Value is: " . $_COOKIE[$cookie_name];
 } 
}
?>

Cheers.