RoomUsage.php

<?php

use Google\Cloud\Datastore\DatastoreClient;

$datastore = new DatastoreClient([
    'projectId' => $projectId
]);

// I aliased $_POST, but you don't need to if you don't want to.
// This is where I'd perform all the validation and sanitation on the data that was given by the user.
$inputData = $_POST;

// Make sure you're setting $roomName somewhere.
$roomName = 'foo';

$name = $roomName . $inputData['timestamp'];

// Create a key with a name.
$key = $datastore->key('RoomUsage', $name);

// We'll create our entity, give it the key we just created, then fill it with data from the form.
$roomUsage = $datastore->entity($key, [
    'Headcount' => ($inputData['countinput'] === '')
        ? $inputData['countslider']
        : $inputData['countinput'],
    'Timestamp' => $inputData['timestamp'],
    'EnteredBy' => $inputData['userid'],
    'RoomID' => $inputData['roomid'],
    'Activiy' => $inputData['activity'],
    'Estimate' => $inputData['estimate'],
    'NotAudited' => $inputData['notaudited'],
    'Reason' => $inputData['reason']
]);

// Finally, send the entity to Datastore.
$datastore->upsert($roomUsage);

Thank you so much for this. It is a thing of beauty.