<?php 
require_once "db_connect.php";
date_default_timezone_set('America/La_Paz');

$producto = $_POST['producto'];

$stmt = $pdo->prepare("SELECT insgpcod, insgpdesc FROM insgp WHERE insgpgrup = ?");
$stmt->execute([$producto]);
$results = $stmt->fetchAll(PDO::FETCH_ASSOC);

$cadsuc = "<option value=''></option>";
foreach ($results as $vesuc) {
   $cadsuc .= '<option value="' . htmlspecialchars($vesuc['insgpcod']) . '">' . htmlspecialchars($vesuc['insgpdesc']) . '</option>';
}
echo $cadsuc;
?>