[xsl] Handling invalid characters

Hi all,

Is there a way to handle/discard invalid characters?

Problem:
I have to use data from several different sources that have
different/unknown encoding types.  I store all of them in a MySQL db
using ISO-8859-1 encoding.  A php function grabs the data and fits it
into XML format.  When I use the XML file as input to an xsl transform,
I get "Invalid character (character code 25) detected".

Possible solutions:
1) I would prefer to somehow convert all invalid characters to valid
ISO-8859-1 (I also have control of the php function)
2) Delete all the invalid characters
3) Display them as gibberish in my documents
4) Basically anything that won't cause an error and will let the
transform continue

Any lead would be much appreciated,
Thanks,
Bao