ถามเรื่อง option selected คับ จะทำยังไงให้มัน ไป select ค้างไว้ที่ บรรทัดที่เรากดเลือก ขอบคุณครับ

if ($key==$selected_sn){
   echo "<option value='$key' selected='selected'>$value</option>\n";
}else{
   echo "<option value='$key'>$value</option>\n";
}

<select size="30" name="SN" id="SN" onchange="submitform()">
          <?php
		foreach($sn_options as $sn_time)
		{
			$a = explode(",",$sn_time);
			$sn = $a[0];
			if (!$selected_sn)
				$selected_sn = $sn;
			if ($sn == $selected_sn)
				echo "\t\t<option selected>$sn</option>\n";
			else
				echo "\t\t<option>$sn</option>\n";
		}
		?>
          </select>

<?php
$array = array(0, 1, 2, 3, 4);
$selected = 2;   # selected 
$list_size = count($array);

echo '<select name="sel" id="sel" size="'.$list_size.'">'."\n";
foreach ($array as $key=>$value) {
	$select_op = ($selected == $key) ?  'selected="selected"' : '';
	echo '<option value="'.$key.'" '.$select_op.'>'.$value.'</option>'."\n";
}
echo '</select>';
?>