您必须在这里传递 $ _ POST ['postid ']

if(isset($_POST['posteraser'])){
   $postid = $_POST['postid'];
   $sql = query("DELETE FROM post_list WHERE postid = '$postid' ");       
   redirect ('home.php');
}

或 b
$ b

OR as procedure way

$sql = query("DELETE FROM post_list WHERE postid = ? ",$postid);