PHP inserting multiple checkbox AND textbox arrays into MySQL Database

Question

I'm having trouble with the array results in my php. My first problem is this :

It shows everything even if the checkbox isn't checked. My second problem is that it won't insert the values to the database even though it's connected to the database (as you can see in the previous screenshot, it says "connected successfully").

This is my html form:

<form method="POST">
    <input type="hidden" name="item[]" value="cupcake">
    <input type="text" name="items" value="cupcake" readonly><br>
    <b>Price :</b> <span name="price" value="3.00">$17.00</span><br>
    Quantity: <input tabindex="1" name="quantity[]" min="0" max="5" type="number" class="quantity" value="1" /><br>
    <input tabindex="1" name="checkbox[]" type="checkbox" value="17" /><span>Add to Cart</span></label></div></div></td><br>

    <input type="hidden" name="item[]" value="cake">
    <input type="text" name="items" value="cake" readonly><br>
    <b>Price :</b> <span name="price" value="20.00">$20.00</span><br>
    Quantity: <input tabindex="1" name="quantity[]" min="0" max="5" type="number" class="quantity" value="1" /><br>
    <input tabindex="1" name="checkbox[]" type="checkbox" value="20" /><span>Add to Cart</span></label></div></div></td><br>

    <input type="submit" name="insertBT"><br>
</form>

PHP:

if(isset($_POST['insertBT']))
{
    class db_conn
    {
        public function create_conn($servername, $username, $password, $db)
        {
            global $conn;
            $conn = new mysqli ($servername, $username, $password, $db);
        }

        public function check_conn()
        {
            global $conn;
            if($conn->connect_error)
            {
                die ("Connection Failed : " . $conn->connect_error);
            }
            else
            {
                echo ("Connected Successfully <br>");
            }
        }

        public function insert()
        {

            if(isset($_POST['checkbox'])) {
                foreach($_POST['checkbox'] as $check) {

                    $check = implode(',', $_POST['checkbox']);
                    $name = implode(',', $_POST['item']);
                    $quantity = implode(',', $_POST['quantity']);
                }
                echo $check . "<br>";
                echo $name . "<br>";
                echo $quantity . "<br>";

                mysql_query("INSERT INTO purchases(Product, Quantity, Price) VALUES('$name', '$quantity','$check')");

            }
        }
    }
    $obj1 = new db_conn;
    $obj1->create_conn("localhost","root","", "dbtest");
    $obj1->check_conn();
    $obj1->insert();
}