how to solve unknown column error in mysql - php

Question

I was working on a forum script when i encounter this error
Notice: Undefined variable: userids in
C:\xampp\htdocs\myfolder\discussion\post_reply_parse.php on line 19
Unknown column 'email' in 'field list'
Please, help me. This is the code
<?php
session_start();
if ($_SESSION['uid']) {
if (isset($_POST['reply_submit'])) {
include_once("connect.php");
$creator = $_SESSION['uid'];
$cid = $_POST['cid'];
$tid = $_POST['tid'];
$reply_content = $_POST['reply_content'];
$sql = "INSERT INTO posts (category_id, topic_id, post_creator, post_content, post_date) VALUES ('".$cid."', '".$tid."', '".$creator."', '".$reply_content."', now())";
$res = mysql_query($sql) or die(mysql_error());
$sql2 = "UPDATE categories SET last_post_date=now(), last_user_posted='".$creator."' WHERE id='".$cid."' LIMIT 1";
$res2 = mysql_query($sql2) or die(mysql_error());
$sql3 = "UPDATE topics SET topic_reply_date=now(), topic_last_user='".$creator."' WHERE id='".$tid."' LIMIT 1";
$res3 = mysql_query($sql3) or die(mysql_error());
$sql4 = "SELECT post_creator FROM posts WHERE category_id='".$cid."' AND topic_id='".$tid."' GROUP BY post_creator";
$res4 = mysql_query($sql4) or die(mysql_error());
while ($row4 = mysql_fetch_assoc($res4)) {
$userids[] .= $row4['post_creator'];
}
foreach ($userids as $key) {
$sql5 = "SELECT id, email FROM users WHERE id='".$key."' AND forum_notification='1' LIMIT 1";
$res5 = mysql_query($sql5) or die(mysql_error());
if (mysql_num_rows($res5) > 0) {
$row5 = mysql_fetch_assoc($res5);
if ($row5['id'] != $creator) {
$email .= $row5['email'].", ";
}
}
}
$email = substr($email, 0, (strlen($email) - 2));
$to = "noreply#somewhere.com";
$from = "YOUR_SITE_EMAIL_HERE";
$bcc = $email;
$subject = "YOUR_SUBJECT_HERE";
$message = "YOU MESSAGE CONTENT HERE";
$headers = "From: $from\r\nReply-To: $from";
$headers .= "\r\nBcc: {$bcc}";
mail($to, $subject, $message, $headers);
if (($res) && ($res2) && ($res3)) {
echo "<p>Your reply has been successfully posted. <a href='view_topic.php?cid=".$cid."&tid=".$tid."'>Click here to return to the topic.</a></p>";
} else {
echo "<p>There was a problem posting your reply. Try again later.</p>";
}
} else {
exit();
}
} else {
exit();
}
?>

Answer 1

Look at the structure of the users table and make sure you use the correct name for the column containing the email address. After that, remove the period on the line
$userids[] .= $row4['post_creator'];
...so it turns into:
$userids[] = $row4['post_creator'];