Barras de progreso con jQuery, Ajax y PHP

<?php
define('DB_SERVER', 'localhost');
define('DB_SERVER_USERNAME', 'USERNAME');
define('DB_SERVER_PASSWORD', 'PASSWORD');
define('DB_DATABASE', 'DATABASE');
 
$connexion = new mysqli(DB_SERVER, DB_SERVER_USERNAME, DB_SERVER_PASSWORD, DB_DATABASE);
?>

<?php 
require('config.php');
$result = $connexion->query('SELECT COUNT(*) as total_comments FROM comments');
$row = $result->fetch_assoc();
$num_total_rows = $row['total_comments'];
 
$update_process = 'UPDATE process SET total = '.$num_total_rows.', percentage = 0, executed = 0, execute_time = "", date_add = now() WHERE id_process = 1';
$connexion->query($update_process);
?>

<form id="start_form" action="#" method="post">
    <input type="hidden" id="total_comments" name="total_comments" value="<?php echo $num_total_rows; ?>" />
    <div class="form-group">
        <label for="batch">Número de elementos que se procesan en cada iteración</label>
        <select class="form-control" id="batch" name="batch">
            <?php
            //divisors
            for($i = 1; $i < $num_total_rows; $i ++) {
                if ($num_total_rows % $i == 0) {
                    echo '<option value="'.$i.'">'.$i.'</option>';
                }
            }
            ?>
        </select>
    </div>
    <div class="form-group">
        <a href="#" class="btn btn-primary" onclick="executeProcess(0);return false;">
            <i class="fa fa-eye"></i> Ejecutar
        </a>
    </div>
</form>

<div id="sending" class="col-lg-12" style="display:none;">
    <h3>Procesando...</h3>
    <div class="progress">
        <div class="progress-bar progress-bar-success progress-bar-striped active" role="progressbar" aria-valuenow="40" aria-valuemin="0" aria-valuemax="100" data-progress="0" style="width: 0%;">
            0%
        </div>
    </div>
    <div class="counter-sending">
        (<span id="done">0</span>/<span id="total">0</span>)
    </div>
 
    <div class="execute-time-content">
        Tiempo transcurrido: <span class="execute-time">0 segundos</span>
    </div>
 
    <div class="end-process" style="display:none;">
        <div class="alert alert-success">El proceso ha sido completado. <a href="https://www.jose-aguilar.com/scripts/jquery/ajax-progress-bar/">Probar otra vez</a></div>
    </div>    
</div>
</div>

<script src="https://code.jquery.com/jquery-3.2.1.js"></script>
<script type="text/javascript">
function executeProcess(offset, batch = false) {
    if (batch == false) {
        batch = parseInt($('#batch').val());
    } else {
        batch = parseInt(batch);
    }
 
    if (offset == 0) {
        $('#start_form').hide();
        $('#sending').show();
        $('#sended').text(0);
        $('#total').text($('#total_comments').val());
 
        //reset progress bar
        $('.progress-bar').css('width', '0%');
        $('.progress-bar').text('0%');
        $('.progress-bar').attr('data-progress', '0');
    }
 
    $.ajax({ 
        type: 'POST',
        dataType: "json",
        url : "process.php", 
        data: {
            id_process: 1,
            offset: offset,
            batch: batch
        },
        success: function(response) {
            $('.progress-bar').css('width', response.percentage+'%');
            $('.progress-bar').text(response.percentage+'%');
            $('.progress-bar').attr('data-progress', response.percentage);
 
            $('#done').text(response.executed);
            $('.execute-time').text(response.execute_time);
 
            if (response.percentage == 100) {
                $('.end-process').show();
            } else {
                var newOffset = offset + batch;
 
                executeProcess(newOffset, batch);
            }
        },
        error: function(XMLHttpRequest, textStatus, errorThrown) {
            if (textStatus == 'parsererror') {
                textStatus = 'Technical error: Unexpected response returned by server. Sending stopped.';
            }
            alert(textStatus);
       }
    });
}
</script>

<?php
require_once('config.php');
 
$offset = $_POST['offset'];
$batch = $_POST['batch'];
 
$result = $connexion->query(
    'SELECT * FROM comments ORDER BY date_added DESC LIMIT '.$offset.', '.$batch
);
if ($result->num_rows > 0) {
    while ($row_comments = $result->fetch_assoc()) {
        //proceso
        $update_comment = "UPDATE comments SET approved = 1 WHERE comment_id = ".$row_comments['comment_id'];
        $connexion->query($update_comment);
 
        $update_process = 'UPDATE process SET executed = executed + 1 WHERE id_process = 1';
        $connexion->query($update_process);
        //sleep(3);
    }
 
    $result_process = $connexion->query('SELECT * FROM process WHERE id_process = 1');
    $row_process = $result_process->fetch_assoc();
 
    $percentage = round(($row_process['executed'] * 100) / $row_process['total'], 2);
 
    $date_add = new DateTime($row_process['date_add']);
    $date_upd = new DateTime($row_process['date_upd']);
    $diff = $date_add->diff($date_upd);
 
    $execute_time = '';
 
    if ($diff->days > 0) {
        $execute_time .= $diff->days.' dias';
    }
    if ($diff->h > 0) {
        $execute_time .= ' '.$diff->h.' horas';
    }
    if ($diff->i > 0) {
        $execute_time .= ' '.$diff->i.' minutos';
    }
 
    if ($diff->s > 1) {
        $execute_time .= ' '.$diff->s.' segundos';
    } else {
        $execute_time .= ' 1 segundo';
    }
 
    $update_process = 'UPDATE process SET percentage = '.$percentage.', execute_time = "'.(string)$execute_time.'" WHERE id_process = 1';
    $connexion->query($update_process);
 
    $row = array(
        'executed' => $row_process['executed'],
        'total' => $row_process['total'],
        'percentage' => round($percentage, 0),
        'execute_time' => $execute_time
    );
    die(json_encode($row));
}
?>