Firstly, my english not good, i am sorry. I hope you will understand me

I should use ISO-8859-9 for my language encode.
I try to use html2pdf project but i get a error.
<?php
require_once("html2pdf.class.php");
$content = "<h2>Some cyrilic characters şiüğçöü</h2>";
$html2pdf = new HTML2PDF("P","A4","tr");
$html2pdf->setEncoding("ISO-8859-9");
$html2pdf->AddFont('Arial','','arial.php');
$html2pdf->pdf->SetFont("Arial");
$html2pdf->WriteHTML($content,false);
$html2pdf->Output("doc.pdf");
?>
If i run this code, i get this error message:
Warning: html_entity_decode() [function.html-entity-decode]: charset `ISO-8859-9' not supported, assuming iso-8859-1 in C:\xampp\htdocs\html2pdf\html2pdf.class.php on line 1961
Warning: Cannot modify header information - headers already sent by (output started at C:\xampp\htdocs\html2pdf\html2pdf.class.php:1961) in C:\xampp\htdocs\html2pdf\_fpdf\fpdf.php on line 1023
FPDF error: Some data has already been output, can't send PDF file
--
Html2PDF do not support ISO-8859-9?
What can i do? Thank you.