XML safe strings and encoding

Hi all,

I'm fighting a bit of a beast here with a string that has fractional
notations in it among other things like quotes etc. Is there a
function that built-in to php that will properly encode this string so
that it is XML safe? I've tried htmlentities and specialchars which
don't work.

Thanks,
vp

PS. I also tried including the bad data in between cdata tags.