The majority of this code works just fine, the database get updated when I click on the button, but the last query (the UPDATE one) doesn't execute for some reason. I tried turning on mysql log on phpmyadmin, but even there it's not executed. It doesn't show me any error, and I really don't know what could be wrong.
$query = "SELECT username, coins FROM users WHERE userid='$userid' LIMIT 1";
$result = mysqli_query($db, $query);
$user = mysqli_fetch_assoc($result);
$_SESSION['username'] = $user['username'];
$_SESSION['coins'] = $user['coins'];
$op = $user['username'];
$op = mysqli_real_escape_string($forumdb, $op);
$postcontent = $_POST['postcontent'];
$postcontent = mysqli_real_escape_string($forumdb, $postcontent);
$posttitle = $_POST['posttitle'];
$posttitle = mysqli_real_escape_string($forumdb, $posttitle);
$sectionid = $_GET['sectionid'];
$sectionid = mysqli_real_escape_string($forumdb, $sectionid);
$query = "INSERT INTO topic (section_id, name, replies, op, lastpost, lastuserid, views, sticked) values('$sectionid', '$posttitle', '0','$op', CURRENT_TIMESTAMP(),'$userid', '0', '0')";
$result = mysqli_query($forumdb, $query) or trigger_error("Query Failed! SQL: $query - Error: ".mysqli_error($forumdb), E_USER_ERROR);
$last_id = mysqli_insert_id($forumdb);
$query = "INSERT INTO posts (topic_id, content, user_id) values('$last_id', '$postcontent', '$userid')";
mysqli_query($forumdb, $query);
$query = "UPDATE section SET lastpost='$username', threads=threads+1, posts=posts+1 WHERE id='$sectionid'";
mysqli_query($forumdb, $query) or trigger_error("Query Failed! SQL: $query - Error: ".mysqli_error($forumdb), E_USER_ERROR);
It would have been helpful if you explained what "doesn't execute for some reason" means.
Either you get an error indicating that the SQL was invalid at runtime or the execution continued and no data was changed.
Without knowing what the error was, we can't advise what would have caused an error. If execution continued, but the record was not (obviously) updated, then it must be because the WHERE clause of the update statement did not match any rows. You could verify this by checking mysqli_affected_rows().
The queries you have run previously wil be in the mysql general log. You might want to echo the SQL statement to the output and check that it is populated as you expect
You can use the following to solve your problem:
"UPDATE `section` SET `lastpost`='$username', `threads`=threads+1, `posts`=posts+1 WHERE `id`='$sectioni