We aggregate the PHP category from del.icio.us on the Planet PHP website. Unfortunately the input from del.icio.us does sometimes contain invalid UTF-8 characters, which leads to errors in the XML parsing. But the following iconv line gets rid of all invalid UTF-8 characters.
$t = iconv("UTF-8", "UTF-8//IGNORE", $t);
Problem solved ;) (But I also wrote a mail to the del.icio.us people about the problem, 'cause this shouldn't happen in the first place)