Send ID from jQuery to a page to make an INQUIRY

I try to explain, since it is something I had never done before:

• Attempt: when opening a modal, I take out an "id" (the one of the client)

• Now, I try to send that "id" to the same page that had made the action, I put the code here:

$(document).on('click', '.editarCliente', function () {

var id = $(this).attr("id");
console.log(id);

$.ajax({
  type: 'POST',
  url: 'php/editar_cliente.php',
  data: {'id': id}
})
.done(function(listas_rep){
  $('#resultEditCliente').html(listas_rep)
})
.fail(function(err){
  console.log(err);
  alert(err);
})

$(document).on('click', '#cerrarEditarCliente', function () {
  $('#editarCliente').hide();
});

});

Why do I use this? Because I try, now on the clients page: create the following query:

<?php  

require_once 'php/conexion.php';

function editarCliente() {

    $idcliente = $_POST['id'];

    $mysqli = getConn();

    $query = "SELECT * FROM clientes WHERE id_cliente = $idcliente";
    $result = $mysqli->query($query);

    $ec = '<form id="form_edit_cliente" method="POST">
            <table id="table" width="100%" class="table table-hover table-mc-light-blue">';

    while($row = $result->fetch_array(MYSQLI_ASSOC)){
        $ec .= '<h2>Editar cliente #'.$row[id_cliente].'</h2>
                <h3>Datos Personales</h3>

                <input id="idc" type="hidden" value="'.$row[id_cliente].'">
                <table>
                    <tr>
                        <td class="campoP">Nombre</td>
                        <td class="td"><input id="nombre" type="text" value="'.$row[nombre].'"></td>
                        <td class="td barcode" style="text-align:center" colspan="6" rowspan="2"></td>
                        <td class="campoP">Fecha Registro:</td>
                        <td class="td">' .$row[fecha_registro]. '</td>
                    </tr>
                    <tr>
                        <td class="campoP">Apellidos</td>
                        <td class="td"><input id="apellidos" type="text" value="'.$row[apellidos].'"></td>
                        <td class="td" colspan="5"></td>
                        <td class="td"></td>
                    </tr>
                    <tr>
                        <td class="campoP">Telefono</td>
                        <td class="td"><input id="telefono" type="text" value="'.$row[telefono].'"></td>
                        <td class="td" colspan="5"></td>
                        <td class="td"></td>
                    </tr>
                    <tr>
                        <td class="campoP">Email</td>
                        <td class="td"><input id="email" type="text" value="'.$row[email].'"></td>
                        <td class="td" colspan="5"></td>
                        <td class="td"></td>
                    </tr>
                </table>
                <h3>Datos Comerciales</h3>
                <table>
                    <tr>
                        <td class="campoP" style="width: 10%">Nombre Comercial</td>
                        <td class="td" style="width: 25%"><input id="ncomercial" type="text" value="'.$row[ncomercial].'"></td>
                        <td class="td" colspan="5"></td>
                        <td class="campoP" style="width: 10%">Nombre Fiscal</td>
                        <td class="td" style="width: 25%"><input id="nfiscal" type="text" value="'.$row[nfiscal].'"></td>
                        <td class="campoP">NIF/CIF</td>
                        <td class="td"><input id="cif" type="text" value="'.$row[cif].'"></td>
                    </tr>
                    <tr>
                        <td class="campoP">Direccion</td>
                        <td class="td"><input id="direccion" type="text" value="'.$row[direccion].'"></td>
                        <td class="td" colspan="5"></td>
                        <td class="campoP">Ciudad</td>
                        <td class="td"><input id="ciudad" type="text" value="'.$row[ciudad].'"></td>
                        <td class="campoP">CP</td>
                        <td class="td"><input id="cp" type="text" value="'.$row[cp].'"></td>
                    </tr>
                </table>';
    }

    $ec .= '</table>
            <br><br>
            <center><input type="submit" id="submit" value="Editar Cliente"></center>
            </form>';

    return $ec;
}

echo editarCliente();
?>

He returns this to me by console:

jquery.min.js:4 POST http://localhost:8888/seoagency/php/editar_cliente.php 500 (Internal Server Error)

By doing this, everything stops working.

Does anyone think of how to help me? Thanks.