Insert dynamic number of checkboxes into many-to-many relationship database

I have a database with a many-to-many relationship with three tables:

+-----------+       +----------+        +----------+
|doorUser   |       |doorAccess|        | doors    |
+-----------+       +----------+        +----------+
| userID    |       | userID   |        | doorNum  |
| lname     |       | doorNum  |        | doorName |
| fname     |       +----------+        +----------+
| username  |
| accessNum |
+-----------+

the doorAccess table is the intermediate table that connects the doorUser and the doors tables. The thing I want to do is to insert into both the doorUser table as well as the doorAccess table, so that each user can have access to any amount of the doors. I have managed to get my PHP to where I can insert into the doorUser table, but the doorAccess table is proving to be a lot more difficult. The below is the snippet from the submits and posts.

if (isset($_POST["submit"])) {
                if( (isset($_POST["fname"]) && $_POST["fname"] !== "") && (isset($_POST["lname"]) && $_POST["lname"] !== "") &&(isset($_POST["username"]) && $_POST["username"] !== "") &&(isset($_POST["accessNum"]) && $_POST["accessNum"] !== "")  ) {

                    $query = "INSERT INTO doorUser ";
                    $query .= "(fname,lname,username,accessNum) ";
                    $query .= "values('".$_POST["fname"]."', '".$_POST["lname"]."', '".$_POST["username"]."', '".$_POST["accessNum"]."')";


                    $query3 = "SELECT doorNum, doorName ";
                    $query3 .= "FROM doors ";

                        $result3 = $mysqli->query($query3);
                            while ($row3 = $result3->fetch_assoc())  {
                                $doorNum = $row3["doorNum"];
                                $doorName = $row3["doorName"];


                            if( (isset($row3["doorName"]) && $row3["doorName"] !== "")){

                                $query3 = "INSERT INTO doorAccess ";
                                $query3 .= "(userID, doorNum) ";
                                $query3 .= "values('".$_POST[LAST_INSERT_ID()]."', '".$_POST["doorNum"]."')"

                                }

                            }                       

And the below is the snippet from the forms.

echo "<p><form action = 'addUser.php?id={$ID}' method='post'>";

                    echo "<p><input type = 'text' name = 'fname' placeholder = 'First Name' /></p>";
                    echo "<p><input type = 'text' name = 'lname' placeholder = 'Last Name' /></p>";
                    echo "<p><input type = 'text' name = 'username' placeholder  = 'username' /></p>";
                    echo "<p><input type = 'text' name = 'accessNum' placeholder = 'password' /></p>";      


                    $query2 = "SELECT doorNum ";
                    $query2 .= "FROM doors ";

                        $result2 = $mysqli->query($query2);
                        if ($result2 && $result2->num_rows > 0) {
                            while ($row2 = $result2->fetch_assoc())  {
                            $doorNum = $row2["doorNum"];

                        echo "<p><input type = 'checkbox' name = 'doorName' value = '".$row2["doorNum"]."' />   Door $doorNum</p>";


                        }
                    }


                    echo "<p><input type = 'submit' name = 'submit' value = 'Add Person' />";   
                    echo "</form>";

The door table is able to be added to or deleted from in another portion, so I need to be able to dynamically generate however many checkboxes for the doors, but then I also need a way to link which users has access to which doors. The checkboxes generate as they're supposed to, but I am having an issue with inserting the userID and the doors into the intermediate table.

I checked and I am able to insert into the doorUser table with this code, but the issues is somewhere in the doorAccess insert.

Any help would be greatly appreciated!