PHP for Web Dev

<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>PHP Cookie Get</title>
</head>
<body>
<?php
ob_start();
 if(isset($_COOKIE['user']))
 {
     $user=$_COOKIE['user'];
     echo "<h1> Welcome $user ! </h1> <hr>";
 }
 else
 {
     echo 'Please <a href="index.html"> Log In </a>';
 }
 ?>

 <a href ="user.php"> option 1</a>
 <a href ="factor.php"> option 2</a>
 <a href ="index.html"> Log out</a>

 <style>
.error {color: #FF0000;}
</style>
</head>
<body>

<h2>Factorial numbering</h2>
<form method="post" action="<?php $_SERVER["PHP_SELF"]; ?>">
  Number: <input type="text" name="Number" value="<?php echo $n;?>">
 <br><br>
  <input type="submit" name="submit" value="Submit">
</form>
<br><br>

<?php
$fact = 1;
$str = "";
$n ;
$error = "";

/* GETTING VALUE FROM IMPUT BOX */

if ($_SERVER["REQUEST_METHOD"] == "POST") {

    if(empty($_POST['Number']))
    {
        $error = "Please enter a number greater than 0";
        echo $error;
    }
    else
    {
        if(ctype_digit($_POST['Number']))
        {
            $n = $_POST['Number'];

           for ($i = $n;$i>=1;$i--):
            $str .= $i.' x ' ;
            $fact *= $i;
        endfor;
        echo "Number Entered: $n <br> Answer:";
        echo trim($str, ' x ') . ' = ' .$fact;

        }
        else
        {
            $error = "Invalid number Entered";
            echo $error;
        }
    }

}

?>

</body>

 </body>
 </html>