在AJAX响应中追踪换行符

I'm doing an AJAX request and somehow a trailing newline is added somewhere.
My PHP script echoes (or is supposed to echo) 'SUCCESS' if the request succeeded, 'ERROR' otherwise.
But currently it returns: 'SUCCESS' (i.e. 'SUCCESS ').
I saw that by adding an alert("!" + msg + "!") that showed the line break.

My AJAX call:

function addMedia() {
    var addMediaName = $("#addMediaName").val();
    var notif;
    if(addMediaName != ""){
        $.ajax({
            url : '../../controler/add/addMedia.php',
            type : 'POST',
            data : "mediaName="+ addMediaName,
            dataType : 'text',
            success: function(msg,data, settings){
                if(msg == 'SUCCESS'){
                    $.toaster({ priority : 'success', title : 'Success', message : 'Mode created' });
                } else {
                    $.toaster({ priority : 'warning', title : 'Failed', message : 'Mode already exists' });
                }
            },
        });
    }
}

My PHP controller:

<?php
include ('../../model/request/add.php');
if((include_once '../../model/request/add.php')===FALSE) exit('erreur include');
$mediaName = $_POST['mediaName'];
$mediaName = ucfirst(strtolower($mediaName));
$media = addMedia($mediaName);
?>

And the addMedia function:

function addMedia($mediaName)
{
    global $conn;
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    try {
        $sql = 'INSERT INTO media (mediaName) VALUES 
        ("'.$mediaName.'")';
        $conn->exec($sql);
        echo 'SUCCESS';
    } catch(PDOException $e) {
        echo 'ERREUR';
    }
}

Any idea where that newline is coming from and how I can fix it?

That is most likely caused by the end of the PHP file, where a newline follows the ?>.

Now while it is possible to die/exit at a previous point, I consider it a much cleaner solution to simply remove the ?>.
The closing tag is not required by PHP, and it is usually even considered better practise not to use it, see this SO question.

add die() or exit() after

echo 'SUCCESS'; die;    

or

echo 'ERREUR'; die;

if you can't add die then code execute next line of code or in this case it will call 'view' in MVC.