使用外键将项插入数据库

I have two different files that deal with this I have a post.php file (url looks like post.php?id=(number)) that deals with the form on the page of the parent message

<form id="chardiv" action="comments.php" method="post">
Name:<br>
<input type="text" name="mod_name" required><br>

Message:<br>

<textarea class='autoExpand' rows='3' data-min-rows='3' placeholder='Auto-Expanding Textarea' name="topic" required></textarea>
<input type="submit" value="Submit"><br>
</form>

and a second file "comments.php" which the html form posts too

<?php
require 'connect.php';
$conn    = Connect();
$id    = (int)$_GET['id'];
$comment   = $conn->real_escape_string($_POST['topic']);
$date    = time();
$query   = "INSERT into anon_comments (post_id,Date,comment) VALUES('" . $id . "','" . $date . "','" . $comment . "')";
$success = $conn->query($query);

if (!$success) {
    die("Couldn't enter data: ".$conn->error);
}
$conn->close();
header('Location: /index.php');
?>

which deals with the posting to the database

I'm unsure how I would go about sending the foreign key (parents id) to the database from the page the form is on.

can someone give aid Thanks!

You are doing wrong here. you have to pass your id in : <input type="hidden" name="id" value="YOUR VALUE" />

and on comments.php page get the value by $id = (int)$_POST['id'];

Hope it helps!!