index.php

<?php @session_start(); ?>

<?if(!isset($_SESSION['fio']) || !($_SESSION['fio'])){?>
    <form method="post" action="set.php" >
            <input type="text" name='fio'>
            <button type="submit">ФИО</button>
    </form>
<?}else{
    print "Привет,".$_SESSION['fio'];
}