How to make a Twitter-like following system - php

Question

How can i make a Twitter-like following system where, if you follow someone, you will receive their tweets. I have a database table called "users", "followings", and "posts". The users table holds the users that are on my site. The followings shows the persons followers, this table is setup like this :'id, user_one, user_two'. And the posts table holds all the tweets or posts that the unique user has.
I have a profile page set up for each user, with a follow button. And also I have a news feed for the user, but in that news feed I need it to show posts from users that the logged-in user is following. So is there anyway for me to have a user follow someone and receive the posts from that user on their news feed.
Now I did some research: their system wouldn't seem to work. I kept getting this error:
Warning: implode() [function.implode]: Invalid arguments passed in function.php on line 12
SELECT
user_id,
body,
stamp
FROM posts
WHERE user_id in ()
ORDER BY stamp DESC
Warning: mysql_fetch_object(): supplied argument is not a valid MySQL result resource in function.php.
This line right here was suppose to help make it where when you follow that person you will receive there tweets and stuff
function show_posts($userid,$limit=0){
$posts = array();
$user_string = implode(',', $userid);
$extra = " and id in ($user_string)";
if ($limit > 0){
$extra = "limit $limit";
}else{
$extra = '';
}
$sql = "select user_id,body, stamp from posts
where user_id in ($user_string)
order by stamp desc $extra";
echo $sql;
$result = mysql_query($sql);
while($data = mysql_fetch_object($result)){
$posts[] = array( 'stamp' => $data->stamp,
'userid' => $data->user_id,
'body' => $data->body
);
}
return $posts;
}
I believe this piece of code, which is supposed to show posts from other users, is causing this problem but I don't know why.

Answer 1

Use a join. For example:
select user_id, text from posts
inner join followings on posts.user_id = followings.followed_id
where followings.follower_id = :user_id;
If I may recommend, stop using the mysql_ functions; they are deprecated and should not be used in newer applications. Instead, I'd recommend using PDO.