// fcm_helper.php
function sendFCMNotification($tokens, $title, $body) {
    $url = 'https://fcm.googleapis.com/fcm/send';
    $serverKey = 'YOUR_SERVER_KEY';

    $headers = [
        'Authorization: key=' . $serverKey,
        'Content-Type: application/json'
    ];

    $data = [
        "registration_ids" => $tokens,
        "notification" => [
            "title" => $title,
            "body" => $body
        ]
    ];

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
    $result = curl_exec($ch);
    curl_close($ch);
    return $result;
}