<?php
if($_POST["nombre"]!="" && $_POST["id_proveedor"]!=""){
    include "../../php/conexion.php";
    $params = array_filter($_POST);

    $id_proveedor = "SELECT id FROM proveedores WHERE nombre = "$_POST[id_proveedor]"";

    $result_id_proveedor = $con->query($id_proveedor);
    $obj = $result_id_proveedor->fetch_object();
    $params['id_proveedor'] = $obj->id_proveedor;

    $params['created_at'] = date("Y-m-d H:i:s");

    $fields = implode(',', array_keys($params));
    $values = '"'.  implode('","',array_values ($params))  . '"';

    $sql = "INSERT INTO marcas ($fields) VALUES ($values)";
    $query = $con->query($sql);
    echo $sql;
    print "<script>alert("Registro exitoso.");window.location='../marcas.php';</script>";
     }

?>

<label for="id_proveedor">Proveedor:</label>
<select class="form-control" name="id_proveedor" id="id_proveedor" required
     <option value="">Seleccione:</option>
     <?php
          include_once "../php/conexion.php";
          $query = $con -> query("SELECT * FROM proveedores");
          while ($valores = mysqli_fetch_array($query)) {
               echo '<option id="id_proveedor" value="'.$valores['id'].'">'.$valores['nombre'].'</option>';
               }
     ?>
</select>