Force show empty value of a table

show.php

<?php
  $conn = mysqli_connect("localhost", "root", "", "test") or die(mysql_error());

  function check_len($arr, $index, $key){
    if(array_key_exists($index, $arr)){
      return $arr[$index][$key];
    }

    return "";
  }

  function show_data($data, $count){
    for($i = 0; $i < $count; $i++){
      echo
        '<tr>'.
          '<td>'. check_len($data, $i, "id_buku") .'</td>'.
          '<td>'. check_len($data, $i, "harga_buku") .'</td>'.
          '<td>'. check_len($data, $i, "penerbit") .'</td>'.
        '</tr>';
    }
  }

  $query = "SELECT id_buku, harga_buku, penerbit FROM buku LIMIT 2";
  $hasil = mysqli_query($conn, $query);

  $output = array();
  while($row = mysqli_fetch_assoc($hasil)){
    $angka = array_push($output, $row);
  }
?>

<table border="1" cellspacing="0" cellpadding="5">
  <?php show_data($output, 3); ?>
</table>