ดึงข้อมูลในฐานข้อมูลมาโชวฺ์ใน listbox ไม่ทราบว่าผิดตรงไหนช่วยดูให้ทีค่ะ

echo "<select name=n_by>";
$sql = "select * from p_tbl_users ";
$res2 = mysql_query($sql);
while($rs2=mysql_fetch_array($res2))
{
	if(trim($rs2[user_name]) == trim($rs2[teach_name]))

	echo "<option value=$rs2[user_name] selected>$rs2[user_name]</option>";
	else
	echo "<option value=$rs2[user_name]>$rs2[user_name]</option>";
}
echo "</select>";
?></td>