if you want to check username and password fields, try below code
if (isset($_POST['submit-btn'])) {
$username = filter_var($_POST['username'], FILTER_SANITIZE_STRING);
$password = filter_var($_POST['password'], FILTER_SANITIZE_STRING);
if(empty($username))
{
echo "Username empty"; return false;
}
if(empty($password))
{
echo "Password empty"; return false;
}
if($username && $password)
{
$login_array = array();
$login_array['user_login'] = $username;
$login_array['user_password'] = $password;
$verify_user = wp_signon($login_array, true);
if (!is_wp_error($verify_user)) {
echo "<script>window.location = '" . site_url() . "'</script>";
} else {
echo 'Credenziali non valide!';
}
}
}
let me know if this works for you!