Using $_GET and $_POST in custom WordPress plugin - php

Question

I'm new to developing plugins for WordPress, I've been doing it for Magento 2 and thought it would be similar, so here I am.
I'm consuming a RESTful API to display information about some courses, it retrieves me the info on all courses so now I have to filter some attributes, for example "city", for this my user has an HTML form to select which city he is in, but when I try to use the "city" field from the form it returns as null, it doesn't matter if I use post or get method.
Which one is the correct form of getting this attributes in WordPress?
this is an example of the form page:
<?php
/*
* Formulario de filtros
*/
// create curl resource
require_once('../../../../wp-config.php');
$ciudad = $_POST['ciudad'];
var_dump($ciudad);
$accessToken = '---';
$apiUrl = 'url';
$curl = curl_init($apiUrl);
curl_setopt($curl, CURLOPT_HTTPHEADER, ['Authorization: Bearer '.$accessToken]);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
$json = curl_exec($curl);
$cursos = json_decode($json);
$arr = 0;
foreach($cursos as $k => $cur)
{
$sede = $cur->Sede;
if($sede === 'MEDELLIN' ){
$codigo[] = substr($cur->Programa, 0, 4);
$programa[] = $cur->Programa;
$precio[] = $cur->Valor_Contado;
}
}
$cod = array_unique($codigo);
var_dump($cod);
foreach ($cod as $c){
echo "Programa: ".$c."<br>";
}
?>
<div class="filtrosContainer">
<form class="filtro-cursos" method="post">
<div class="vc_row wpb_row vc_row-fluid edgtf-section edgtf-content-aligment-left"><div class="clearfix edgtf-full-section-inner"><div class="wpb_column vc_column_container vc_col-sm-6"><div class="vc_column-inner "><div class="wpb_wrapper">
<div class="wpb_text_column wpb_content_element ">
<div class="wpb_wrapper">
<label>Metodología</label>
<select name="metodologia" id="metodologia" class="metodologia">
<option selected>--Seleccione--</option>
<option value="PRESENCIAL" required>Presencial</option>
<option value="VIRTUAL">Virtual</option>
</select>
<div id="ciudadDiv">
<label>Ciudad</label>
<select name="ciudad" id="ciudad">
<option value="none" selected>--Seleccione--</option>
<option value="MEDELLIN" required>Medellín</option>
<option value="BOGOTA">Bogotá</option>
<option value="BARRANQUILLA">Barranquilla</option>
<option value="BUCARAMANGA">Bucaramanga</option>
<option value="MANIZALES">Manizales</option>
<option value="CALI">Cali</option>
</select></div>
<label>Programa</label>
<select name="programa" id="programa">
<?php
if($ciudad === "MEDELLIN"){ ?>
<option value="none" selected>--Seleccione--</option>
<option value="none" selected>PreUdeA</option>
<option value="none" selected>PreUnal</option>
<option value="none" selected>PreICFES Saber 11°</option>
<option value="none" selected>PreMédico</option>
<option value="none" selected>PreUdeA</option>
<?php }else{
echo $ciudad;
}
?>
</select>
</div>
</div>
</div></div></div><div class="wpb_column vc_column_container vc_col-sm-6"><div class="vc_column-inner "><div class="wpb_wrapper">
<div class="wpb_text_column wpb_content_element ">
<div class="wpb_wrapper">
<label>¿Cuando presentarás el examen?</label>
<select name="semestre" id="semestre">
<option value="none" selected>--Seleccione--</option>
<option value="2019-1" selected>2019-1</option>
<option value="2019-2" selected>2019-2</option>
</select>
<div id="jornadaDiv">
<label>Jornada</label>
<select name="jornada" id="jornada">
<option value="none" selected>--Seleccione--</option>
<option value="SEMANA" selected>Semana</option>
<option value="FINDE" selected>Fin de semana</option>
</select>
</div>
<button type="submit" name="submit" value="Filtrar">Filtra tu programa</button>
</div>
</div>
</div></div></div></div></div>
</form>
</div>

Answer 1

After some research and trying several ways I tryed out the code all together in the main PHP file of my custom plugin instead of using the require_once.
I had the next estructure:
wp-content>>plugins>>myplugin>>myplugin.php
and in said php file had a require_once for:
wp-content>>plugins>>myplugin>>php>>form.php
it seems you can't use $_POST, $_GET, not evev cookies in the files outside the main php file so I had to work all my code in there and forget about mu php folder and eny MVC structure.