ใช้ for วนลูป เพื่อในค่าใน textbox ไปเก็บไว้ใน session โดยเก็บแบบ array ครับ

<?
session_start(); 
for ( $j=1;$j<2;$j++ )
  {
    for($i=0;$i<count($_POST["addons_select_sub$j"]);$i++)
        {
            $_SESSION["addons_select_sub_count$j"] = count($_POST["addons_select_sub$j"]);
            $addons_select_sub."".$j = $_POST['addons_select_sub$j'][$i];

                if ($addons_select_sub."".$j != '')
                    {
                            $_SESSION["addons_select_sub$j$i"] = $addons_select_sub."".$j;
                    }
        }
  }
session_write_close();
?>


<form name="item" method="post" action="" ENCTYPE = "multipart/form-data" onsubmit="return checkform(this);">
<input type="text" name="addons_select_sub1[]">
<input type="text" name="addons_select_sub1[]">
<input type="text" name="addons_select_sub1[]">

<input type="text" name="addons_select_sub2[]">
<input type="text" name="addons_select_sub2[]">
<input type="text" name="addons_select_sub2[]">
<input type="submit" name="submit" value="ok">
</form>

<?php
    session_start();
   foreach ($_SESSION as $key=>$val)
    echo $key." ".$val."<br/>";
?>