Vanilla php scipt to send emails via X-Mailer

mailer.php

<?php

$db_host = 'hostname';
$db_user = 'username';
$db_password = 'password';
$db_name = 'database';

$mysqli = new mysqli($db_host, $db_user, $db_password, $db_name);

if ($mysqli->connect_errno) {
    if($debug)
    {
        echo "Error: " . $mysqli->connect_error;
    }
    return;
}

if (!$mysqli->set_charset("utf8")) {
    if($debug)
    {
        echo "Encoding error: " . $mysqli->error();
    }
    return;
}

$debug = true;

if($_SERVER['REQUEST_METHOD'] === 'GET')
{
    header('Location: /');
}
else if($_SERVER['REQUEST_METHOD'] === 'POST')
{
    
    if(isset($_POST['botcheck']) && strlen($_POST['botcheck']))
    {
        echo 'bot GTFO';
        return;
    }
    
    if(!(isset($_POST['email']) || isset($_POST['phone'])))
    {
        if($debug)
        {
            echo 'GTFO';
        }
        return;
    }
    
    $name = isset($_POST['name']) ? htmlspecialchars($_POST['name']) : '-';
    $question = isset($_POST['question']) ? htmlspecialchars($_POST['question']) : '-';
    
    $email = (isset($_POST['email'])) ? htmlspecialchars($_POST['email']) : '-';
    $campaign = (isset($_POST['campaign'])) ? htmlspecialchars($_POST['campaign']) : '-';
    $phone = (isset($_POST['phone'])) ? htmlspecialchars($_POST['phone']) : '-';
    
    $res = $mysqli->query("INSERT INTO `requests`(`name`,`email`,`phone`,`campaign`,`date`) VALUES('$name', '$email', '$phone', '$campaign', '" . date('Y-m-d') . "')");
    
    if($res)
    {
        echo 'INSERT OK';
    }
    else
    {
        echo 'INSERT NE OK';
        echo $mysqli->error;
    }
    
    $headers = 'From: Name<[email protected]>' . "\r\n" .
        'Reply-To: [email protected]' . "\r\n" .
        'X-Mailer: PHP/' . phpversion();
    $message = 'Новая заявка:' . "\r\n";
    
    if($name !== '-')
    {
        $message .= 'Имя: ' . $name . "\r\n";
    }
    if($phone !== '-')
    {
        $message .= 'Телефон: ' . $phone . "\r\n";
    }
    if($email !== '-')
    {
        $message .= 'Email: ' . $email . "\r\n";
    }
    if($question !== '-')
    {
        $message .= "Вопрос/пожелание:\r\n" . $question . "\r\n";
    }
    
    $mails = [
        "[email protected]",
        "[email protected]",
    ];
    
    foreach($users as $user)
    {
        mail($user, 'Новая заявка Инвестум клуб', $message, $headers);
    }
    
    $mysqli->close();
}