Send SQL concat to PHP variable - php

Question

I am trying to select multiple columns with concat an put the returned data into one textbox.
I think there is something wrong with my definition for the variables. But I could not figured out what is wrong. Here are the variables:
$id = isset($_POST['id'])?$_POST['id']:'';
$name = isset($_POST['firstname'])?$_POST['firstname']:'';
$name .= isset($_POST['insertion'])?$_POST['insertion']:'';
$name .= isset($_POST['lastname'])?$_POST['lastname']:'';
When I define just one variable for $name the script works. But that is not what I want.
Does someone know what is wrong?
Here is the other part of my script.
First I have a textbox. The data needs to be send to this textbox:
<input type="text" class="form-control" id="name" name="name" placeholder="Name">
The button calls sends '5' as the ID and runs the script getName():
<button type="button" rel="5" onclick="getName();"
<script type="text/javascript">
$('body').on('click', '.selectClass', function () {
var id = $(this).attr('rel');
$("#id").val(id);
modal.style.display = "none";
});
</script>
After clicking on the button the id is deployed here:
<input type="text" class="form-control" id="id" name="id">
The onClick event runs the following script:
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script>
function getName(value) { // Do an Ajax request to retrieve the product price
console.log("getName before ajax", jQuery('#id').val());
jQuery.ajax({
url: './get/getname5.php',
method: 'POST',
data: {'id' : jQuery('#id').val()},
success: function(response){
console.log("getName after ajax", jQuery('#id').val());
jQuery('#name').val(response);
},
error: function (request, status, error) {
alert(request.responseText);
},
});
}
</script>
The jquery script calls the PHP, which is not working with the multiple variables for $name
<?php
session_start();
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "db";
$conn = new mysqli($servername, $username, $password, $dbname) ;
if ($conn->connect_error) {
die('Connection failed: ' . $conn->connect_error) ;
}else {
$id = isset($_POST['id'])?$_POST['id']:'';
$name = isset($_POST['firstname'])?$_POST['firstname']:'';
$name .= isset($_POST['insertion'])?$_POST['insertion']:'';
$name .= isset($_POST['lastname'])?$_POST['lastname']:'';
$query = 'SELECT concat(firstname, ' ', insertion, ' ', lastname) as name FROM users WHERE id="' . mysqli_real_escape_string($conn, $id) . '"';
$res = mysqli_query($conn, $query) ;
if (mysqli_num_rows($res) > 0) {
$result = mysqli_fetch_assoc($res) ;
echo $result['name'];
}else{
$result = mysqli_fetch_assoc($res) ;
echo $result['name'];
}
}
?>