如何解決php echo亂碼問題

<?php
header("Content-Type:text/html;charset=gb2312");
//設置頁面字符集
$conn=mysql_connect("localhost", "root", "****");
//****為mysql密碼
mysql_select_db("world");
mysql_query("set names utf8");
$sql="select * from city order by population desc";
$res=mysql_query($sql);
echo "<h1>城市信息一覽表</h1>";
echo "<table width='1000px'>";
echo "<tr>";
echo "<td>id</td><td>name</td><td>countrycode</td><td>district</td><td>population</td>";
echo "</tr>";
while($row=mysql_fetch_assoc($res)){
 echo "<tr>";
 echo "<td>{$row['ID']}</td><td>{$row['Name']}</td><td>{$row['CountryCode']}</td><td>{$row['District']}</td><td>{$row['Population']}</td>";
 echo "</tr>";
}
echo "</table>";
mysql_close($conn);
?>