Concatenate input values html/php
<form action="./ucp.php?mode=login" method="post">
    <fieldset>
        <!-- my new input boxes -->
        <label for="username">First name:</label>&nbsp;
        <input type="text" name="firstname" id="firstname" size="10" title="Firstname" ><br><br>
        <label for="username">Last name:</label>&nbsp;
        <input type="text" name="lastname" id="lastname" size="10" title="Lastname" ><br><br>

        <!-- original input box; now hidden -->
        <label for="username">Username:</label>&nbsp;
        <input type="hidden" name="username" id="username" size="10" title="Username" />

        <label for="password">Password:</label>&nbsp;
        <input type="password" name="password" id="password" size="10" title="Password" />

        <br><label for="autologin">Log me on automatically each visit
        <input type="checkbox" name="autologin" id="autologin" /></label>

        <br><input type="submit" name="login" value="Login" />
    </fieldset>
</form>

onsubmit="this.username.value=this.firstname.value+this.lastname.value; return true;";

$user = $_POST['firstname'].$_POST['lastname'];

if($_GET['mode'] == 'login')
$username=  $_POST['firstname'].$_POST['lastname'];