<?php
if (empty($_SERVER['HTTP_X_UP_SUBNO'])){
    $id = md5($_SERVER['HTTP_USER_AGENT']);
} else {
    $id = md5($_SERVER['HTTP_X_UP_SUBNO']);
}
session_id($id);
 
session_start();
echo "<a href=\"2.php\">○○サイトへ</a><br>";
print_r($_SESSION);
?>

<?php
if (empty($_SERVER['HTTP_X_UP_SUBNO'])){
    $id = md5($_SERVER['HTTP_USER_AGENT']);
} else {
    $id = md5($_SERVER['HTTP_X_UP_SUBNO']);
}
session_id($id);
 
session_start();
$_SESSION["auth"] = "YES";
header("Location: http://hoge.com");
?>