Questions to send data via POST

This is the code for my Form:

<div class="container-fluid">
<div class="panel panel-primary">
    <div class="panel-heading">
        CADASTRAR USUÁRIO
    </div><br>

    <div id="filtros" style="padding-left: 20px">
        <a id="voltar_pagina" href="menu.php" class="btn btn-primary" title="Voltar ao início">
            <span class="glyphicon glyphicon-home"></span>
        </a>
        <a id="voltar_lista" href="javascript:history.back()" class="btn btn-primary" title="Voltar">
            <span class="glyphicon glyphicon-chevron-left"></span>
        </a>                        
    </div>

    <div class="panel-body">

        <form id="form_cad_usuario" data-toggle="validator" class="form-horizontal" method="POST" action="../config/usuario/processa_cad_usuario.php">

            <!--DADOS DO USUÁRIO-->
            <div class="page-header">
                <h4>Dados do Usuário</h4s>
                <hr>
            </div>


            <div class="form-group">                 
                <label for="ativo" class="col-sm-2 control-label">Ativo</label>
                <div class="col-sm-10">
                    <div class="checkbox">
                        <label>
                            <input id="ativo" type="checkbox">
                        </label>
                    </div>
                </div>
            </div>

            <div class="form-group">
                <label for="nome_usuario" class="col-sm-2 control-label">Nome</label>
                <div class="col-sm-10">
                    <input type="text" class="form-control" id="nome_usuario" placeholder="Nome" data-error="Por favor, informe o nome completo para o usuário." required>
                    <div class="help-block with-errors"></div>
                </div>
            </div>

            <!--DADOS DE ACESSO-->
            <div class="page-header">
                <h4>Dados de Acesso</h4>
                <hr>
            </div>

            <div class="form-group">
                <label for="login_usuario" class="col-sm-2 control-label">Usuário</label>
                <div class="col-sm-10">
                    <input type="text" class="form-control" id="login_usuario" placeholder="Usuário" data-error="Por favor, digite um nome para o usuário." required>
                    <div class="help-block with-errors"></div>
                </div>
            </div>

            <div class="form-group">
                <label for="senha_usuario" class="col-sm-2 control-label">Senha</label>
                <div class="col-sm-10">
                    <input type="password" class="form-control" id="senha_usuario" placeholder="Senha" data-error="Por favor, digite um senha para o usuário." required>
                    <div class="help-block with-errors"></div>
                </div>
            </div>

            <!--DADOS DA  REGIONAL-->
            <div class="page-header">
                <h4>Regionais Permitidas</h4>
                <hr>
            </div>

            <?php
            $resultado = lista_regionais();

            while ($registro = mysqli_fetch_assoc($resultado)) 
            {
                $id_regional = $registro['id'];
                $nome_regional = $registro['nome'];

            ?>

            <div class="col-sm-2">
                <div class="form-group">
                    <div class="col-sm-1">
                        <div class="checkbox">
                            <label>
                                <label class="col-md-10" for="<?php echo"regional_$id_regional" ?>">
                                    <input type="checkbox" name="<?php echo"regionais[$id_regional]"; ?>" id="<?php echo"regional_$id_regional" ?>"> 
                                    <small><?php echo $nome_regional; ?></small>
                                </label>
                            </label>
                        </div>
                    </div>
                </div>
            </div>

            <?php
            }
            ?>

            <div class="form-group">
                <div class="col-sm-offset-2 col-sm-10">
                    <button id="botao_salvar" type="submit" class="btn btn-primary pull-right">Salvar <span class="glyphicon glyphicon-floppy-save"></span></button>
                </div>
            </div>

        </form>
    </div>
</div>

Here is the script 'processa_cad_user.php':

<?php
session_start();

include_once ('../../dao/usuario_dao.php');

$_SESSION['cadastro_inserido'] = null;

var_dump($_POST['nome_usuario']);

//cadastra_usuario($nome, $login, $senha, $permissao, $flag_ativo, $criado, $modificado);

When I try to access, for example $ _ POST ['user_name'] php tells me that such index is not defined and by doing var_dump () returns me Null. Any suggestions?

Additionally I would like to remedy an extra doubt. At the end of the form I have some checkboxes with names of cities that are generated dynamically and I would like to know how to pass all that are selected to the page that processes the register and how should I insert them into the database and which structure use. This keeping in mind that at some point I will need them to give the user permission to access data from the cities for which he has permission. Thank you immensely to anyone who can help. Hugs to all.