if(isset($_POST['checkbox']))
{
foreach ($_POST['checkbox'] as $value){
$checkbox= $_POST['checkbox'];
$n = count($checkbox);
$i = 0;
}
while ($i < $n)
{
$id_query = "INSERT into tbl(obj_id, category_id) VALUES ('$id_obj', '$checkbox[$i]]')";
mysql_query($id_query) or die(mysql_error());
$i++;
}
}