Form:

echo "<form action='forums.php' method='post'>";
echo "<input type='hidden' name='forum' value='" . $_GET['forum'] . "'>";
echo "Subject: <input type='text' style='width: 95%;' name='subject' /><br />";
echo "<textarea style='width: 95%;' rows='10' name='content'></textarea><br />";
echo "<input type='submit' name='newthread' value='Post' /><br />";
echo "</form>";

Catcher:

if (isset($_POST['newthread'])) {
    $posted = date("Y-m-d H:i:s", time());
    $poster = getval("users", "id", "username", $_SESSION['user']);
    $post = $mysqli->real_escape_string(stripslashes($_POST['content']));
    $title = $mysqli->real_escape_string(stripslashes($_POST['subject']));
    echo "<!-- Variables: ";
    print_r($_POST);
    echo "-->";
    $result = $mysqli->query("INSERT INTO posts (content, posted, poster, forum) VALUES (" . $post . ", $posted,  $poster, " . $_POST['forum'] . ")"); //Line 316
    $result2 = $mysqli->query("INSERT INTO threads (forum, title, posted, poster, views, posts, lastposttime) VALUES (" . $_POST['forum'] . ", " . $_POST['subject'] . ", $posted, $poster, 1, 0, $posted)");
    $thread = $mysqli->query("SELECT id FROM threads WHERE title='$title' AND forum=" . $_POST['forum'] . " AND posted=$posted AND poster=$poster")->fetch_row();
    $post = $mysqli->query("SELECT id FROM posts WHERE posted=$posted AND poster=" . $_SESSION['user'] . " AND forum=" . $_POST['forum'])->fetch_row();
    $result3 = $mysqli->query("UPDATE posts SET thread=" . $thread[0] . " WHERE id=" . $post[0]);
    if ($result && $result2 && $result3) {
        echo "Your post has been created.<br /><br />";
        $res = $mysqli->query("SELECT id FROM posts WHERE posted=$posted AND thread=" . $_POST['thread'] . " AND poster=$poster")->fetch_row();
        echo "<a href='forums.php?thread=" . $_POST['thread'] . "#post" . $res[0] . ">Click here</a> to go to your post.<br />";
        echo "<a href='forums.php?thread=" . $_POST['thread'] . ">Click here</a> to return to the previous thread.";
    } else {
        $mysqli->query("DELETE FROM posts WHERE posted=$posted AND poster=$poster AND forum=" . $_POST['forum']);
        $mysqli->query("DELETE FROM threads WHERE forum=" . $_POST['forum'] . " AND title=" . $_POST['subject'] . " AND posted=$posted AND poster=$poster");
        echo "Error creating post: " . $mysqli->error . "<br /><br />";
        echo "<a href='forums.php?thread=" . $_POST['thread'] . ">Click here</a> to return to the previous thread.";
    }
}

Output:

Notice: Undefined index: forum in /home/tristan/Website/Instart/forums.php on line 316 Notice: Undefined index: forum in /home/tristan/Website/Instart/forums.php on line 317 Notice: Undefined index: forum in /home/tristan/Website/Instart/forums.php on line 318 Fatal error: Call to a member function fetch_row() on a non-object in /home/tristan/Website/Instart/forums.php on line 318

Content of the comment (filled with a print_r):

<!-- Variables: Array
(
    [subject] => Next milestone.
    [content] => This is the first thread created through the website, rather than through the database.
    [newthread] => Post
)
-->