how to insert foreign key into a database table using php

I need help, I haven't used foreign key in a very long time and I cant remember how too. I can insert data into the database without the foreign key into the table, but I am unable to insert data into the database with the foreign key. I have provided my code in hope that someone is able to help me. thank you.

$error_msg = "";

if (isset($_POST['firstname'], $_POST['lastname'], $_POST['homephone'], $_POST['cellphone'], $_POST['address'], $_POST['city'], $_POST['state'], $_POST['zip']))

{

// Sanitize and validate the data passed in

$firstname = filter_input(INPUT_POST, 'firstname', FILTER_SANITIZE_STRING);

$lastname = filter_input(INPUT_POST, 'lastname', FILTER_SANITIZE_STRING);

$homephone = filter_input(INPUT_POST, 'homephone', FILTER_SANITIZE_STRING);

$cellphone = filter_input(INPUT_POST, 'cellphone', FILTER_SANITIZE_STRING);

$address = filter_input(INPUT_POST, 'address', FILTER_SANITIZE_STRING);

$city = filter_input(INPUT_POST, 'city', FILTER_SANITIZE_STRING);

$state = filter_input(INPUT_POST, 'state', FILTER_SANITIZE_STRING);

$zip = filter_input(INPUT_POST, 'zip', FILTER_SANITIZE_STRING);

}

if (empty($error_msg))

{

// Insert the members info into the database

if ($insert_stmt = $mysqli->prepare("INSERT INTO members_info (members_fk, firstname, lastname, homephone, cellphone, address, city, state, zip) VALUES (( SELECT * FROM members WHERE id = '" . $_SESSION['username'] . "' ), ?, ?, ?, ?, ?, ?, ?, ?)"))

{

$insert_stmt->bind_param('sssssssss', $last_insert_id(), $firstname, $lastname, $homephone, $cellphone, $address, $city, $state, $zip);

// Execute the prepared query.

if (! $insert_stmt->execute())

{

header('Location: ../error.php?err=Registration failure: INSERT');

exit();

}

}

header('Location: ../member_profile.php');

exit();

}