輸入後立即顯示在同一網頁，供您參考
<?
include "Link.php";
if (isset($_REQUEST["action"]) && $_REQUEST["action"] != ""){
     $action = $_REQUEST["action"];
}else{
     $action = "";
}
if ($action == "insert"){
  $xxx = $_REQUEST["xxx"];
  $db->query("INSERT INTO test (xxx) VALUES ('$xxx')");
}else{
?>
<form method='post' action='send_msg.php' name='write_msg'>
<INPUT type="hidden" name="action" value="insert">
<input type='text' maxlength=ཐ' size=ཐ' name='xxx'>
</form>
<?
  $rows = $db->query("SELECT * FROM test");
  while ($row=$db->fetch_array($rows)){
    echo $xxx;
  }
}
?>

感謝教學
雖然我還不太會"->"的用法，但靠著

<INPUT type="hidden" name="action" value="insert">

if (isset($_REQUEST["action"]) && $_REQUEST["action"] != ""){
          $action = $_REQUEST["action"];
      }else{
          $action = "";
      }

就成功的解決我的問題了