Ajax / Json DataTables

Question

I have 3 files, index.php, ajax.php, and scripts.js.

This is my ajax.php:

if ( $_GET['action'] == "newUserAccountsGrid" )
{

    try
    {

        $stmt = $DB_CONN->prepare("SELECT * FROM crm_users");
        $stmt->execute();
        $i=1;
        while ( $rows = $stmt->fetch(PDO::FETCH_OBJ) )
        {
            $obj[$i]['username']        = $rows->{'username'};
            $obj[$i]['firstname']       = $rows->{'firstname'};
            $obj[$i]['lastname']        = $rows->{'lastname'};
            $obj[$i]['user_email']      = $rows->{'user_email'};
            $obj[$i]['user_level']      = $rows->{'user_level'};
            $obj[$i]['user_role']       = $rows->{'user_role'};
            $obj[$i]['date_created']    = $rows->{'date_created'};
            $i++;
        }

    }
    catch( PDOException $e )
    {
        echo $e->getMessage();
    }

    if ( is_array($obj) )
    {
        $arr['data'] = array_values($obj);
        $json = json_encode($arr, JSON_PRETTY_PRINT);
        echo $json;
    }

}

datatable javascript scripts.js :

$( "#tbl_useraccounts" ).dataTable({
    ajax: "ajax.php?action=newUserAccountsGrid",
    columns: [
        { "data": "username" },
        { "data": "firstname" },
        { "data": "lastname" },
        { "data": "user_email" },
        { "data": "user_level" },
        { "data": "user_role" },
        { "data": "date_created" },
    ],

    columnDefs: [{
        defaultContent: "-",
        targets: "_all"
    }]
});

Also index.php :

<table id="tbl_useraccounts" class="table table-striped table-hover">
    <thead>
        <tr>
            <th class="no-sort">Username</th>
            <th class="no-sort">First Name</th>
            <th class="no-sort">Last Name</th>
            <th class="no-sort">User Email</th>
            <th class="no-sort">User Level</th>
            <th class="no-sort">User Role</th>
            <th class="no-sort">Date Created</th>
        </tr>
    </thead>
    <tfoot>
        <tr>
            <th>Username</th>
            <th>First Name</th>
            <th>Last Name</th>
            <th>User Email</th>
            <th>User Level</th>
            <th>User Role</th>
            <th>Date Created</th>
        </tr>
    </tfoot>
</table>

Json is ok :

{
    "data": [
        {
            "username": "test1",
            "firstname": "Test 1",
            "lastname": "Test 1",
            "user_email": "[e-mail]",
            "user_level": "1",
            "user_role": "Broker",
            "date_created": "2017-06-19 15:10:32"
        },
        {
            "username": "test2",
            "firstname": "Test 2",
            "lastname": "Test 2",
            "user_email": "[e-mail]",
            "user_level": "1",
            "user_role": "Consulent",
            "date_created": "2017-06-19 15:10:32"
        },
        {
            "username": "test3",
            "firstname": "Test 3",
            "lastname": "Test 3",
            "user_email": "[e-mail]",
            "user_level": "1",
            "user_role": "Consulent",
            "date_created": "2017-06-19 15:10:32"
        }
    ]
}

So everything executes fine, no errors, table renders the rows, but no data is displayed. Thank you guys for your time.