Twitter API - Reply to a retweet

I am writing a little twitter API (using Elliot Haughin's library) and I want to reply to a retweet that the app posts.

I have read the docs for statues/update, and have tried using '*in_reply_to_status_id*' with the value taken from '*user_mentions*', but it doesn't seem to work.

What am I doing wrong, or is there a better way to do it?

Code sample;

$answer = '@'.$retweet->entities->user_mentions[0]->screen_name.' thanks!!';
 $this->tweet->call('post', 'statuses/update', array('status' => $answer, 'in_reply_to_status_id' => $retweet->id));
 

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