Untitled

form1.php
###########

<form action="form2.php" method="POST">
    <select name="userid">
        <option value="1">User 1</option>
        <option value="2">User 2</option>
        <option value="3">User 3</option>
    </select>
    <input type="submit" name="submit" value="Submit">
</form>

form2.php
###########

<?php if (ISSET($_POST['submit']) && !empty($_POST['userid'])): ?>
<html>
<body>
<form action="form3.php" method="POST">
    <input type="hidden" name="userid" value="<?= $_POST['userid'];?>">
    <input type="text" name="your_other_input[]">
    <input type="text" name="your_other_input[]">
    <input type="text" name="your_other_input[]">
    <input type="submit" name="submit" value="Submit">
</form>
</body>
</html>

<?php else: ?>
<?php exit(header("location: form1.php")); ?>
<?php endif; ?>