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[Marco van Herwaarden]

In the following code, $threadid is an array with the contents of a single database row.

$threadid = $vbulletin->db->query_first("
       SELECT threadid
       FROM " . TABLE_PREFIX . "thread
       WHERE pollid = '".$pollida."'
   ");

So to use the threadid in the next query, you will need to use $threadid['threadid']:

$checkpost = $vbulletin->db->query_read("
       SELECT username
       FROM " . TABLE_PREFIX . "post
       WHERE threadid = ".$threadid['threadid']."