As part of the Integration of DirtMind with twitter I needed to be able to submit posts into my phpbb3 forum using a bot account to protect anonymity of my users when they’re tweeting their DirtyThoughts anonymously, and I didn’t want to use the anonymous/guest account because I wanted to generate hit counts etc. Here’s how I did it

        $poll = $uid = $bitfield = $options = '';   
        generate_text_for_storage($twitter_status, $uid, $bitfield, $options, true, true, true);

        $data = array(
           'forum_id'      => 11, //the subforum that my DirtyThoughts thread is in
           'icon_id'      => false,
           'topic_id'      => 435, //the thread to reply to
           'poster_id'      => 146, //the id of the DirtyThoughts user              

           'enable_bbcode'   => true,
           'enable_smilies'   => true,
           'enable_urls'      => true,
           'enable_sig'      => true,

           'message'      => $twitter_status,
           'message_md5'   => md5($twitter_status),

           'bbcode_bitfield'   => $bitfield,
           'bbcode_uid'      => $uid,

           'post_edit_locked'   => 0,
           'topic_title'      => '',
           'notify_set'      => false,
           'notify'         => false,
           'post_time'       => 0,
           'forum_name'      => '',
           'enable_indexing'   => true,          
        );          
        //store current user
        $old_user_id=$user->data['user_id'];
        $old_username=$user->data['username'];       
        $old_usercol=$user->data['user_colour'];       
        //change to bot
        $user->data['username']='DirtyThoughts';
        $user->data['user_id']= 146;
        $user->data['user_colour']= '';
        //make post
        submit_post('reply', 'Dirty Thought', 'DirtyThoughts', POST_NORMAL, $poll, $data, '');    
        //restore user
        $user->data['username']=$old_username;
        $user->data['user_id']= $old_user_id;
        $user->data['user_colour'] = $old_usercol;

Interestingly the submit_post function always uses the current user object to make a post, regardless of the settings in $data. To get around this I store the current userid, name and colour in variables  and switch to the bot values to make the post before returning the current users settings to their normal values.