php problem

in a huge php page it's happen that some page included in index.php, for some strange reason went write. 2 byte are write on the pages included, and are always the same (FF and FE in hexadecilam) at the start of file. so the file will be FF FE<? php blablabla ?> i cannot understand why, i don't use fopen, fwrite, and things like that, so what i can do to debug this script? also i cannot understand where the problem is, so i don't know if is regarded to a page included in index.php or in some other place. Any help is appreciated! best regards Matteo