<?php

// Тrial...

session_start();

if(empty($_POST)) {

$num1 = rand(0,9);

$num2 = rand(0,9);

$num3 = rand(0,9);

$num4 = rand(0,9);

$num = "$num1$num2$num3$num4";

$_SESSION['num'] = $num;

echo $_SESSION['num'];

echo '<form action="index.php?num" method="post">

<input type="text" name="code"><input type="submit" value="Ввести"></form>';

} elseif(isset($_GET['num'])) {

$code = $_SESSION['num'];

$num = $_POST['code'];

if($code == $num) {

echo 'верно';

} else {

echo 'не верно';}}

?>