PHP - php BD

<?php // Probar la base de datos classics

  require_once 'conectar.php';

  if (isset($_POST['delete']) && isset($_POST['isbn']))

  {

    $isbn   = get_post($conn, 'isbn');

    $query  = "DELETE FROM classics WHERE ISBN='$isbn'";

    $result = $conn->query($query);

  	if (!$result) echo "Error al tratar de borrar: $query<br>" .

      $conn->error . "<br><br>";

  }

  if (isset($_POST['author'])   &&

      isset($_POST['title'])    &&

      isset($_POST['category']) &&

      isset($_POST['year'])     &&

      isset($_POST['isbn']))

  {

    $author   = get_post($conn, 'author');

    $title    = get_post($conn, 'title');

    $category = get_post($conn, 'category');

    $year     = get_post($conn, 'year');

    $isbn     = get_post($conn, 'isbn');

    $query    = "INSERT INTO classics VALUES" .

      "('$author', '$title', '$category', '$year', '$isbn')";

    $result   = $conn->query($query);

  	if (!$result) echo "Error al tratar de insertar: $query<br>" .

      $conn->error . "<br><br>";

  }

  echo <<<_END

  <form action="probar-classics.php" method="post">

  <pre>

    Autor    <input type="text" name="author">

    Título   <input type="text" name="title">

    Tema     <input type="text" name="category">

    Año      <input type="text" name="year">

    ISBN     <input type="text" name="isbn">

           <input type="submit" value="Agregar Registro">      <input type="reset" value="Limpiar campos">

  </pre></form>

_END;

  $query  = "SELECT * FROM classics";

  $result = $conn->query($query);

  if (!$result) die ("Falló conexión a la base de datos: " . $conn->error);

  $rows = $result->num_rows;

  for ($j = 0 ; $j < $rows ; ++$j)

  {

    $result->data_seek($j);

    $row = $result->fetch_array(MYSQLI_NUM);

    echo <<<_END

  <pre>

    Autor      $row['Autor']

    Título     $row['Título']

    Tema       $row['Tema']

    Año        $row['Año']

    ISBN       $row['isbn']

  </pre>

  <form action="probar-classics.php" method="post">

  <input type="hidden" name="delete" value="yes">

  <input type="hidden" name="isbn" value="$row[4]">

  <input type="submit" value="Borrar Registro"></form>

_END;

  }

  $result->close();

  $conn->close();

  function get_post($conn, $var)

  {

    return $conn->real_escape_string($_POST[$var]);

  }

?>