禁用文本字段

this is my code. this code will fetch somethings from my database. this not our matter. i have one or more than one check boxes. if in my while i have only one record there is no problem but when my records are more that one checkboxes no act.

here is the script of disable textfield via checkbox

<script language="JavaScript">
<!--

function enable_text(status)
{
status=!status; 
    document.f1.explain.disabled = status;
}
//-->
</script>  

  <?php
    $id = $fgmembersite->UserID(); 
    $db_host = 'localhost';
    $db_name = 'site';
    $db_table= 'action';
    $db_user = 'root';
    $db_pass = '';
    $con = mysql_connect($db_host, $db_user, $db_pass) or die("خطا در اتصال به پايگاه داده");
    $selected = mysql_select_db($db_name, $con) or die("خطا در انتخاب پايگاه داده");
    mysql_query("SET CHARACTER SET utf8");

    $dbresult = mysql_query("
        SELECT tablesite.name,
               tablesite.family,
               tablesite.username,
               tablesite.phone_number,
               tablesite.email,
               action.service_provider_comment,
               action.price,
               action.date,
               job_list.job_name,
               action.ind
          FROM $db_table
         INNER JOIN job_list ON job_list.job_id = action.job_id 
         INNER JOIN tablesite ON tablesite.id_user=action.service_provider_id AND action.customer_id='$id'
                           WHERE vote = ''", $con);
       $i = 1;

       while ($amch = mysql_fetch_assoc($dbresult)) {
           echo "<form name=f1 id='form_$i' method='post' action='{$_SERVER['PHP_SELF']}' accept-charset='UTF-8'>
";
           echo'<div dir="rtl">';
           echo "نام خدمت دهنده: " . "&nbsp&nbsp&nbsp".$amch["name"] . " " . $amch["family"] . "&nbsp&nbsp&nbsp" . "شماره تماس: " . $amch["phone_number"]. "&nbsp&nbsp&nbsp" . "ایمیل: " . $amch["email"] . '<br/>' .
    "شغل انجام شده: ".$amch["job_name"] . '<br/>'
    ."تاریخ انجام عملیات: ".$amch["date"].'<br/>'
    ."هزینه ی کار: ".$amch["price"]." تومان".'<br/>'
    .$amch["service_provider_comment"].'<hr/>';
        echo'<label for="explain" style="color:#CC0000">این فیلد را فقط در صورتی پر کنید که شکایت دارید</label> <br />';
        echo'<input type="checkbox" name=others onclick="enable_text(this.checked)" >شکایت دارم <br>'; //here is checkbox
        echo'<textarea name="explain" id="explain" cols="" rows="" style="width:300 ;height:300"> //checkbox can disable or enable this field
        </textarea>'.'<br/>'; 

    echo '<input type="hidden" name="ind" value="' . $amch["ind"] . '">'; //new line
    echo '<br/>';
    echo '<input type="submit" name="submit" value="ارسال نظر شما"/>';echo'<hr/>';
    echo'<hr/>';
    echo'</div>';
    echo "</form>
";
        $i++;
    }
    ?>