Try this link.
http://www.php.net/manual/function.imagettftext.php
(Assuming that a truetypefont for thai characters exists)
The command imagestring can only use the system fonts of your operating system. With imagettftext you can use any TrueTypeFont (which you have to copy to your webserver!)
Attention: PHP must be installed with the TTF-Library, otherwise this command doesn't work.
good look