การส่งค่าใน checkbox ไปยัง textarea ช่วยทีครับผม php

<br />

  <table width="357" border="1" align="center" cellpadding="0" cellspacing="0" bordercolor="#999999">
    <tr>
	  <td width="39" height="25" align="center" bgcolor="#66FF99">เลือก</td>
      <td width="114" bgcolor="#66FF99">เบอร์โทร</td>
      <td colspan="2" bgcolor="#66FF99">ชื่อ</td>
    </tr>
    <? while ($recs<=$pagesize){
		$result=mysql_fetch_array($q_dbquery);
		$txtu_id=$result[mem_tel];
		if ($txtu_id){?>
  <tr>
     <td width="39" align="center"><input type="checkbox" name="id_array[]" value="<?=$txtu_id?>" /> </td>
     <td bgcolor="#99CCFF"><?=$txtu_id?><input type="hidden" name="a<?=$txtu_id?>" id="txtid" value="<?=$txtu_id?>," /></td>
      <td width="160" bgcolor="#99CCFF"><?=$result[mem_name]?> <?=$result[mem_suname]?></td>
    </tr>
    <? }  $recs++;}?>
</table>
      <? }   ?>  
         <td><input type="submit" name="btnSubmit" value="Submit"    onClick="window.close();"/>
  <script language="JavaScript">
<!-- Begin
document.form1.txtid.focus();
// End -->
  </script>