Example code for PHP class

wifi-password.php

<?php

// DATA
$passwords = [    
    "John" => "1357", 
    "Susan" => "2468",
    "Steven" => "1890",
    "Steve" => "9876"
];

// LOGIC
$lastname = $_POST["lastname"] ?? "";
$password = $passwords[$lastname] ?? "";

// INTERFACE
?>

<h1>Welcome <?= $lastname ?>.</h1>
<?php if ($password != ""): ?>
 <p>Your Wi-Fi password is <?= $password ?>.</p>
<?php endif; ?>

<form method="POST" action="">
    Please enter your last name:
    <input type="text" name="lastname">
    
    <input type="submit">
</form>