simple number guessing via php session

<?php

function start_again(){
        $_SESSION['num'] = $number = rand(1, 50);
    $_SESSION['attempt'] = 0;
    echo '<p>Guess new number!</p>';
}

if (!isset($_SESSION)){
    session_start();
}

var_export($_SESSION); echo '<br/>';

?><!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Number Guessing Game!</title>
</head>
<body>
    <?php
    $userInput = $_POST['number'];
    if(!isset($_SESSION['num'])){
    start_again();
    }
    else{
        $number = $_SESSION['num'];
        $attempt = $_SESSION['attempt'];
    if ($number==$userInput) { $message = 'Bingo!'; }
    if ($number<$userInput) { $message = 'random number is smaller!'; $_SESSION['attempt']++; }
    if ($number>$userInput) { $message = 'random number is bigger!'; $_SESSION['attempt']++; }
        echo "you entered $userInput <br/>$message <br>";
    }

    echo "<h3> $number / $attempt </h3>"; //just to see if the number is changing everytime

   if ($number==$userInput) { start_again(); }

?>
    <form method="post" action="<?php echo $_SERVER["PHP_SELF"];?>">
        <input type="text" name="number" value="" /> <input type="submit" name="Submit"
            value="Submit" />
    </form>
</body>
</html>