for ($i=0; $i < $len; $i++){

  if($book[$i]["bookedFor"]!=$book[$i+1]["bookedFor"]){    //ILLEGAL OFFSET HERE

    echo "<tr>";
    echo "<td>" . $book[$i]["roomName"] . "</td>";
    echo "<td>" . $book[$i]['bookedFor'] . "</td>";
    echo "<td>";
    for ($j=0; $j < $len; $j++){

        if($book[$i]["bookedFor"]==$book[$j]["bookedFor"]){

            if(!empty($book[$j]['resourceName'])){

                if($book[$i]["bookedFor"]!=$book[$j+1]["bookedFor"]){    //ILLEGAL OFFSET HERE

                    echo $book[$j]['resourceName'];

                } else {

                    echo $book[$j]['resourceName'] . ", ";
                }
            }
        }
    }
    echo "</td>";
    echo "</tr>";
  }
}

if (!array_key_exists($i+1, $book)) {
    $book[$i+1] = "Closed";
}