```php
<?php
header("Content-Type: application/json");
header("Access-Control-Allow-Origin: *");
header("Access-Control-Allow-Methods: GET, POST, OPTIONS");
header("Access-Control-Allow-Headers: Content-Type");

// Database configuration
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "tokigunesparkevleri_db";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
    die(json_encode(["error" => "Connection failed: " . $conn->connect_error]));
}

// Handle different API endpoints
$request = $_SERVER['REQUEST_URI'];
$method = $_SERVER['REQUEST_METHOD'];

// Contact form submission
if ($method === 'POST' && strpos($request, '/api/contact') !== false) {
    $data = json_decode(file_get_contents('php://input'), true);
    
    $name = $data['name'] ?? '';
    $email = $data['email'] ?? '';
    $phone = $data['phone'] ?? '';
    $message = $data['message'] ?? '';
    
    if (empty($name) || empty($email) || empty($message)) {
        http_response_code(400);
        echo json_encode(["error" => "Missing required fields"]);
        exit;
    }
    
    $stmt = $conn->prepare("INSERT INTO contacts (name, email, phone, message, created_at) VALUES (?, ?, ?, ?, NOW())");
    $stmt->bind_param("ssss", $name, $email, $phone, $message);
    
    if ($stmt->execute()) {
        echo json_encode(["success" => "Message sent successfully"]);
    } else {
        http_response_code(500);
        echo json_encode(["error" => "Failed to send message"]);
    }
    
    $stmt->close();
    exit;
}

// Get Instagram posts
if ($method === 'GET' && strpos($request, '/api/instagram') !== false) {
    $result = $conn->query("SELECT * FROM instagram_posts ORDER BY post_date DESC LIMIT 6");
    
    $posts = [];
    while ($row = $result->fetch_assoc()) {
        $posts[] = [
            'id' => $row['id'],
            'image_url' => $row['image_url'],
            'author' => $row['author'],
            'author_image' => $row['author_image'],
            'content' => $row['content'],
            'likes' => $row['likes'],
            'posted_at' => $row['post_date']
        ];
    }
    
    echo json_encode($posts);
    exit;
}

// Newsletter subscription
if ($method === 'POST' && strpos($request, '/api/newsletter') !== false) {
    $data = json_decode(file_get_contents('php://input'), true);
    $email = $data['email'] ?? '';
    
    if (empty($email) || !filter_var($email, FILTER_VALIDATE_EMAIL)) {
        http_response_code(400);
        echo json_encode(["error" => "Invalid email address"]);
        exit;
    }
    
    $stmt = $conn->prepare("INSERT INTO newsletter_subscribers (email, subscribed_at) VALUES (?, NOW())");
    $stmt->bind_param("s", $email);
    
    if ($stmt->execute()) {
        echo json_encode(["success" => "Subscribed successfully"]);
    } else {
        http_response_code(500);
        echo json_encode(["error" => "Subscription failed"]);
    }
    
    $stmt->close();
    exit;
}

// Default response for unknown endpoints
http_response_code(404);
echo json_encode(["error" => "Endpoint not found"]);
$conn->close();
?>
```