arduino.php

<?php

$users = array(
    'f884624f' => 'Jonathan',
);

if (!isset($_GET['tag'])) {
    echo '0:No';
    die();
}

$received = $_GET['tag'];

if (array_key_exists($received, $users)) {
    echo '1:Hi ' . $users[$received];
} else {
    echo '0:NoUser';
}