php wont let me echo JSON string - php

Question

Throught an AJAX call I want to pick some data from DB and send back to client.
$result is return value of SELECT QUERY
Case1:This works fine
$str = "[";
while($row = mysqli_fetch_assoc($result)) {
$str = $str ."{".
'"Name":'. '"'. $row["Name"] . '"'
. "},";
}
}
$str = $str . "]";
echo str;
CASE2:This works fine too:
$str = "[";
while($row = mysqli_fetch_assoc($result)) {
$str = $str ."{".
'"ID":'. '"'. $row["ID"] //SEE HERE
. "},";
}
} else {
echo "0 results";
}
$str = $str . "]";
echo $str;
CASE3: BUT FOLLOWING GIVES error with 500()
$str = "[";
while($row = mysqli_fetch_assoc($result)) {
$str = $str ."{".
'"ID":'. '"'. $row["ID"] . '",' //SEE HERE
'"Name":'. '"'. $row["Name"] . '"'
. "},";
}
} else {
echo "0 results";
}
$str = $str . "]";
echo $str;
I want to send echo str after making it in JSON format and parse it at client side using `JSON.parse(). Case1 and 2 work fine when I have only one key-value pair. But as soon as I put 2 key-value pairs I get error:
POST https://******************.php 500 ()

Answer 1

You really need to use the appropriate functions.
Get into good programming habits.
$rtn = []; //create array
while($row = mysqli_fetch_assoc($result)){
//push result into array
$rtn[] = ['ID'=> $row["ID"], 'Name'=> $row['Name']];
}
if($rtn)//if array has entries
echo json_encode($rtn);//encodes result to json
else
echo "0 results";

Answer 2

It return 500, because you have error in your script. You can add below code and check where is the mistake.
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
Probably in your code is missing dot. Check this:
$str = $str . "{" .
'"ID":' . '"' . $row["ID"] . '",' .
'"Name":' . '"' . $row["Name"] . '"'
. "},";