為何這個(gè)alert出來的刪除成功，還是亂碼？

<?php
header("Content-type: text/html; charset=utf-8"); 
require('conn.php');
$id = intval($_GET['id']);
$sql = "delete from lyb where ID = $id";
if ($mysqli->query($sql) === true) {
    echo "<meta http-equiv='Content-Type'  content='text/html; charset=utf-8'>"; 
    echo "<script charset='utf-8' type='text/javascript'>alert('刪除成功');location.href='5-6.php';</script>";
} else {
    echo "刪除失敗" . $mysqli->error;
    echo  "<script>location.href='5-6.php'</script>";
}
$mysqli->close();
?>

為何刪除成功幾個(gè)字，還是顯示為亂碼？