matrix_id, user_id, position_1, position_2, position_3
   1          1        1982        2251       5841
   2          2        6204
   3          3
   4          4

$find_user = $db->prepare("SELECT * FROM matrix WHERE user_id > :user_id");
$find_user->bindValue(':user_id', 0);
$find_user->execute();
$result_user = $find_user->fetchAll(PDO::FETCH_ASSOC);
if(count($result_user) > 0) {
  foreach($result_user as $row) {
    $matrix_id              = $row['matrix_id'];
    $user_id                = $row['user_id'];
    $position_1               = $row['position_1'];
    $position_2               = $row['position_2'];
    $position_3               = $row['position_3'];
  }
} else {
  $errors[] = 'User Id not found in Matrix.';
}

$update_user = $db->prepare("UPDATE matrix SET position_2 = :position_2 WHERE user_id = :user_id");
$update_user->bindValue(':position_2', 1564;
$update_user->bindParam(':user_id', $user_id);
if($update_user->execute()) {}