PHP JSON generation problems - php

Question

The code below should send back an json with informations in a Database.
It takes two parameters grade and subject. The problem ist, when I use parameters are not in the database behind everything works as expected no entry, but if it would get an answer from the database nothing appears. I mean really nothing. The values i need are there i tried this and no errors are logged into the logging file. As server runs apache2 with php5.6.22 on Debian. I don't know what i did wrong. Hopefully someone can help me.
The Code:
case 'get_books':
$grade = $_GET['grade'];
$subject = $_GET['subject'];
$sqlt = "SELECT * FROM book_type WHERE subject=".$subject." AND grade=".$grade;
$sql = mysqli_query($db, $sqlt);
if(!$sql){
print(json_encode(array('response' => 2)));
die();
}
$response = array();
$response['books'] = array();
while($row=mysqli_fetch_assoc($sql)) {
$book = array();
$book['fullname'] = $row ["fullname"];
$book['ISBN'] = $row ["ISBN"];
$book['id'] = $row ["id"];
array_push($response['books'], $book);
}
$response['response'] = "1";
print(json_encode($response));
die();

Answer 1

I think this might be your problem:
array_push($response['books'], $book);
as far as I know you can't push a variable into a specific index of an array since no key is provided for the item being pushed.
It would be better to do this as follows:
case 'get_books':
$grade = $_GET['grade'];
$subject = $_GET['subject'];
$sqlt = "SELECT * FROM book_type WHERE subject=".mysqli_real_escape_string((htmlspecialchars_decode($subject, ENT_QUOTES)))." AND grade=".mysqli_real_escape_string((htmlspecialchars_decode($grade, ENT_QUOTES)));
$sql = mysqli_query($db, $sqlt);
if(!$sql){
print(json_encode(array('response' => 2)));
die('sql failed');
}
$response = array();
$response['books'] = array();
$response['validator'] = 'valid';
$i = 0;
while($row=mysqli_fetch_assoc($sql)) {
$book = array();
$book['fullname'] = $row["fullname"];
$book['ISBN'] = $row["ISBN"];
$book['id'] = $row["id"];
$response['books'][$i] = $book;
$i++;
}
$response['response'] = "1";
var_dump($response);
//echo json_encode($response);
die();