View file zip0.ru/messages_unread_count.php

<?php
require_once 'db.php';
header('Content-Type: application/json; charset=utf-8');

if (empty($_SESSION['user_id'])) {
    echo json_encode(['count' => 0]);
    exit;
}

$user_id = (int)$_SESSION['user_id'];

$stmt = $mysqli->prepare("
    SELECT COUNT(*)
    FROM messages
    WHERE to_user_id = ? AND is_read = 0
");
$stmt->bind_param('i', $user_id);
$stmt->execute();
$stmt->bind_result($cnt);
$stmt->fetch();
$stmt->close();

echo json_encode(['count' => (int)$cnt]);