here the logic i'm thinking.
This isn't actual php code.
user logs in -> give them a session(username)
user replies to post -> give them a session(postid) or cookie(postid) or store it in db
now, when the user goes back to the post.
If (session(postid) == CurrentPostIDofPage){
echo "you can't post anymore today/ or for the session";
}
else{
// show html to reply to post.
}
that should give you an idea.
