我找不到在特定PDO实例中提交此复选框循环结果的方法

I've tried many ways to get these checkboxes loop result and include it in a SQL table through PDO, I'm out of ideas and the console doesn't show any error too. Here's the HTML and the AJAX + PHP part. The SQL column details: 'occupations' VARCHAR 50 utf8_general_ci.

<form action="send.php" method="post" id="newsletter" name="newsletter">
  <p class="newsletter-disclaimer" style="font-size: 25px; margin: 30px 0 10px 0;">Please, select your occupation(s)</p>
  <div class="checkbox-recording-artist">
    <div class="form-check form-check-inline">
      <input class="form-check-input" type="checkbox" id="inlineCheckbox1" value="Recording Artist" name="occupations_list[]">
      <label class="form-check-label" for="inlineCheckbox1">Recording Artist</label>
    </div>
  </div>
  <div class="checkbox-music-producer">
    <div class="form-check form-check-inline">
      <input class="form-check-input" type="checkbox" id="inlineCheckbox2" value="Music Producer" name="occupations_list[]">
      <label class="form-check-label" for="inlineCheckbox2">Music Producer</label>
    </div>
  </div>
  <div class="checkbox-dj">
    <div class="form-check form-check-inline">
      <input class="form-check-input" type="checkbox" id="inlineCheckbox3" value="DJ or Business" name="occupations_list[]">
      <label class="form-check-label" for="inlineCheckbox3">DJ or Business</label>
    </div>
  </div>
  <p class="newsletter-disclaimer" style="font-size: 25px; margin: 20px 0 0 0;">✉ What's your best email?</p>
  <p class="newsletter-disclaimer">We'll never send spam.</p>
  <div id="response"></div>
  <input type="email" name="signup-email" id="signup-email" value="" placeholder="✉ Enter your email address..." autocomplete="email" required />
  <button type="submit" name="submit" class="btn btn-white">Enter VIP List Now<img src="assets/img/arr-b.png"></button>
</form>

How can I include the checkboxes loop result into the table in the same instance as "email" and "datetime"?

Here's the PHP file linked to the form method.

<?php
    require 'inc/Database.class.php';
    require 'inc/Newsletter.class.php';
    if (!empty($_POST)) {
        $email = $_POST['signup-email'];
        if(!empty($_POST['occupations_list'])){
            foreach($_POST['occupations_list'] as $occupations){
                echo $occupations."</br>";
            }
        }
        Newsletter::register($email);       
    }
    ?>

Here's the database + pdo part, which I'm confused how to get these checkboxes loop result to that same "email" and "datetime" instance because the field "occupations" is directly linked to the visitor's email and datetime of submission.

<?php

    class Newsletter
    {
        private static $email;
        private static $datetime = null;        
        private static $valid = true;

        public function __construct() {
            die('Init function is not allowed');
        }

        public static function register($email) {
            if (!empty($_POST)) {
                self::$email    = $_POST['signup-email'];
                self::$datetime = date('Y-m-d H:i:s');

                if (empty(self::$email)) {
                    $status  = "error";
                    $message = "The email address field must not be blank";
                    self::$valid = false;
                } else if (!filter_var(self::$email, FILTER_VALIDATE_EMAIL)) {
                    $status  = "error";
                    $message = "You must fill the field with a valid email address";
                    self::$valid = false;
                }

                if (self::$valid) {
                    $pdo = Database::connect();
                    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
                    $existingSignup = $pdo->prepare("SELECT COUNT(*) FROM signups WHERE signup_email_address='$email'");
                    $existingSignup->execute();
                    $data_exists = ($existingSignup->fetchColumn() > 0) ? true : false;

                    if (!$data_exists) {
                        $sql = "INSERT INTO signups (signup_email_address, signup_date) VALUES (:email, :datetime)";
                        $q = $pdo->prepare($sql);

                        $q->execute(
                            array(':email' => self::$email, ':datetime' => self::$datetime));

                        if ($q) {
                            $status  = "success";
                            $message = "Success! Welcome to the VIP List. If you don't receive an email from us in 3 minutes please check your spam folder and mark our emails as \"NOT SPAM\"";
                        } else {
                            $status  = "error";
                            $message = "<i class=\"fas fa-exclamation-triangle\"></i>An error occurred, please try again";
                        }
                    } else {
                        $status  = "error";
                        $message = "<i class=\"fas fa-exclamation-triangle\"></i> You are already a VIP member of the list.";
                    }
                }

                $data = array(
                    'status'  => $status,
                    'message' => $message
                );

                echo json_encode($data);

                Database::disconnect();
            }
        }

    }

</div>