saveHTMLFile generates empty files

Hi all I have problems with non ascii characters. There is no problem if the client sends UTF-8 encoded strings. But I don't own all clients ;-) Is there a possibility to check errors before I save the File? In the examle I try to change a HTML file. If you give öäü (German Umlauts) as GET Parameter the html File is destroyed. The saveHTMLFile gives a warning but that's too late. There is an other problem with the save method. If the Harddisk is full, the save method converts to a delete method ;-) Bernhard test.html: ========== <html> <body> </body> </html> php program: ============ <?php $dom=domDocument::loadHTMLFile('test.html'); $body=$dom->getElementsByTagName("body")->item(0); $textnode=$dom->createTextNode($_GET['umlaut']); //try öäü $body->appendChild($textnode); print $dom->saveHTML(); //save changed html file //if(file is ok and has no iconv errors){ <-- how to do that? $dom->saveHTMLFile('test.html') //} ?>