Inserting values with a foreign key relationship into two different tables simultaneously? (MySQL)

Question

So I have 2 tables:

• users with columns id (primary, auto_increment), username, password, person_id (foreign key)
• people with columns id (primary, auto_increment), first_name, last_name

What I'm trying to do is when registering a new account have a new row inserted into people and then have a new row inserted into users with the people.id as foreign key users.person_id.

Right now I have 2 php functions that get executed right after eachother, firstly one with this query:

insert into people (first_name, last_name) values (:firstname, :lastname)

Secondly one with this query:

insert into users (username, password, person_id) values (:user, :pass, LAST_INSERT_ID())

All of this works fine except for the fact that last_insert_id() keeps giving value 0 instead of the id from the previous query. Is it maybe not possible to use last_insert_id() when using 2 separate queries? If so what would be the best way to go about it then?

This is my relevant php code:

//make new person
$newPerson = new PeopleManagement();
$pm = $newPerson->createNewPerson($_POST["firstName"], $_POST["lastName"]);

//make new user
$newUsr = new Authentication();
$ac = $newUsr->registerNewUser($_POST["user"], $_POST["pass"]);


public function registerNewUser ($user, $pass) {
    try {

        $dbm = new PDO(DBCONFIG::$db_conn, DBCONFIG::$db_user, DBCONFIG::$db_pass);
        $dbm->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
        $dbm->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);        

            hash = password_hash($pass, PASSWORD_DEFAULT);

            $sql = "insert into users (username, password, person_id) values (:user, :pass, LAST_INSERT_ID())";

            $stmt = $dbm->prepare($sql);
            $stmt->execute(array(
                ':user' => $user,
                ':pass' => $hash
            ));

            $dbm = null;

    } catch(PDOException $ex) {
        return "Could not connect to database";
    }
}

public function createNewPerson($firstName, $lastName) {

    $dbm = new PDO($this->dbConn, $this->dbUser, $this->dbPass);

    $sql = "insert into people (first_name, last_name) values (:firstname, :lastname)";

    $stmt = $dbm->prepare($sql);
    $stmt->execute(array(
        ':firstname' => $firstName,
        ':lastname' => $lastName
    ));

    $dbm = null;
}