$count = $db->prepare("
        select * from
        (select count(id) from img where `mod` = '0' and block = '0' and category = :id and time = :time) as count1,
        (select count(id) from img where `mod` = '0' and block = '0' and category = :id) as count2
    ");
$count = $count->execute(array(
    ':id' => $post['id'],
    ':time' => time()-86400
));

$count = $count->fetch(PDO::FETCH_OBJ);

# первый
echo $count->count1;

# второй
echo $count->count2;