<form action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']). '?post=' . $index->actualPrefix(); ?>" method="post" id="comentarios">
            <textarea name="comments" id="comments"></textarea>
            <input type="submit" value="Comment">
<?php if (!empty($errores)): ?>
            <div class="error">
                    <ul>
                        <?php echo $errores; ?>
                    </ul>
            </div>
<?php endif; ?>
        </form>

if($_SERVER['REQUEST_METHOD'] == 'POST'){
    $article = $index->actualPrefix();
    $user = $index->limpiarDatos($_SESSION["id_user"]);
    $comment = $index->limpiarDatos($_POST['comments']);
    if (empty($comment)) {
        $errores .= '<li>An empty comment was sent, rejected</li>';
    }
    if ($errores == '') {
        $conexion = new Conexion();
        $statement = $conexion->prepare(
            'INSERT INTO comment (id_article, id_user, content) VALUES (:id_article, :id_user, :content)'
        );
        $statement->execute(array(
            ':id_article' => $article,
            ':id_user' => $user,
            ':content' => $comment
        ));
    }
}