Simple subgrid is way to display quick a tabular data in master detail. In this case the detail data is just a information and nothing more. You can not sort, reorder and etc. Look into 2 level subgrid in order to do this.

The hierarchy is achieved in this example with adding another file and tell the main grid from where to get this data.
All is done with just additional 4 lines of code.
<?php
require_once '../../../tabs.php';
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
  <head>
    <title>jqGrid PHP Demo</title>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../themes/redmond/jquery-ui-1.8.2.custom.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../themes/ui.jqgrid.css" />
    <link rel="stylesheet" type="text/css" media="screen" href="../../../themes/ui.multiselect.css" />
    <style type="text">
        html, body {
        margin: 0;            /* Remove body margin/padding */
        padding: 0;
        overflow: hidden;    /* Remove scroll bars on browser window */
        font-size: 75%;
        }
    </style>
    <script src="../../../js/jquery.js" type="text/javascript"></script>
    <script src="../../../js/i18n/grid.locale-en.js" type="text/javascript"></script>
    <script type="text/javascript">
    $.jgrid.no_legacy_api = true;
    $.jgrid.useJSON = true;
    </script>
    <script src="../../../js/jquery.jqGrid.min.js" type="text/javascript"></script>
    <script src="../../../js/jquery-ui-custom.min.js" type="text/javascript"></script>
  </head>
  <body>
      <div>
          <?php include ("grid.php");?>
      </div>
      <br/>
      <?php tabs(array("grid.php","subgrid.php"));?>
   </body>
</html>
grid.php.
<?php
require_once '../../../jq-config.php';
// include the jqGrid Class
require_once ABSPATH."php/jqGrid.php";
// include the driver class
require_once ABSPATH."php/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES utf8");

// Create the jqGrid instance
$grid = new jqGridRender($conn);
// Write the SQL Query
$grid->SelectCommand 'SELECT CustomerID, CompanyName, ContactName, Phone, City FROM customers';
// Set the table to where you update the data
$grid->table 'customers';
// Set output format to json
$grid->dataType 'json';
// Let the grid create the model
$grid->setPrimaryKeyId('CustomerID');
$grid->setColModel();
// Set the url from where we obtain the data
$grid->setUrl('grid.php');
// Set some grid options
$grid->setGridOptions(array(
    
"rowNum"=>10,
    
"height"=>250,
    
"gridview"=>false,
    
"rowList"=>array(10,20,30),
    
"sortname"=>"CustomerID"
));
$grid->setColProperty('CustomerID', array("label"=>"ID""width"=>50));
// Set the parameters for the subgrid
$grid->setSubGrid("subgrid.php",
        array(
'OrderID''RequiredDate''ShipName''ShipCity''Freight'),
        array(
60,120,150,100,70),
        array(
'left','left','left','left','right'));
// Enable navigator
$grid->navigator true;
// Enable only editing
$grid->setNavOptions('navigator', array("excel"=>false,"add"=>false,"edit"=>false,"del"=>false,"view"=>false));
// Enjoy
$grid->renderGrid('#grid','#pager',truenullnulltrue,true);
$conn null;
?>
subgrid.php.
<?php
require_once '../../../jq-config.php';
// include the jqGrid Class
require_once ABSPATH."php/jqGrid.php";
// include the driver class
require_once ABSPATH."php/jqGridPdo.php";
// Connection to the server
$conn = new PDO(DB_DSN,DB_USER,DB_PASSWORD);
// Tell the db that we use utf-8
$conn->query("SET NAMES utf8");
// Get the needed parameters passed from the main grid
$rowid jqGridUtils::GetParam("id");
if(!
$rowid) die("Missed parameters");
// Create the base jqGrid instance
$grid = new jqGrid($conn);
// Write the SQL Query
$grid->SubgridCommand "SELECT OrderID, RequiredDate, ShipName, ShipCity, Freight FROM orders WHERE CustomerID=? ORDER BY OrderID desc";
// set the ouput format to json
$grid->dataType 'json';
// Use the build in function for the simple subgrid
$grid->querySubGrid(array(&$rowid));
$conn null;
?>