<form method=post action=/path/to/page2.php>
First name: <input type=text name=fname value=""><br>
<form method=post action=/path/to/signup.php>
<input type=hidden name=fname value="<?php echo htmlspecialchars($_POST['fname']); ?>
">
<form method=post action=/path/to/page2.php onSubmit="if(!document.getElementById('fn').value){ alert('Please enter a first name'); return false; }">
First name: <input type=text id=fn name=fname value=""><br>