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// check_age.php
<?php
session_start();
if (isset($_POST['submit'], $_POST['adult']) && $_POST['adult'] == 1) {
    $_SESSION['adult'] = 'true';
    header ('location: index.php');
}
?>
<html>
<head></head>
<body>
<form action="" method="POST">
<input type="checkbox" name="adult" value="1">
I am over 21 years old
<input type="submit" name="submit">
</form>
</body>
</html>

// has_age.php
<?php
session_start();
if (!isset($_SESSION['adult'])) {
header('location: check_age.php');
}
?>

// index.php

// first line

require_once 'has_age.php';