Invalid tokens

Good morning!

I am working with PHP, expat (Apache mod) and I am having some
difficulty with XML that I receive from an outside source. When parsing
I get an error message, "not well-formed (invalid token)". I have
searched the web for days now trying to find a way to eliminate the
error messages so that the XML file will be parsed properly. 

I am unsure if this is the right list for this question, but if anyone
can point me in the right direction I would surely appreciate it.

TIA!

Jay