How can PHP be used to automatically display upgrade notifications to users based on download IDs and user IDs?

// Check if user is eligible for an upgrade based on their user ID and download ID
$user_id = 123; // User ID
$download_id = 456; // Download ID

// Check if user has purchased the download
if(check_download_purchase($user_id, $download_id)){
    // Display upgrade notification to user
    echo "Upgrade available for download ID: " . $download_id;
}

// Function to check if user has purchased the download
function check_download_purchase($user_id, $download_id){
    // Query database to check if user has purchased the download
    // Replace this with your actual database query
    $purchased_downloads = array(123, 456, 789); // Example list of purchased downloads for user ID 123
    if(in_array($download_id, $purchased_downloads)){
        return true;
    } else {
        return false;
    }
}