Encode non-ASCII char for RSS/XML output

For example, “, ”...

[iProgram]

I need to generate XML feed of some articles and some of this articles contain non-ASCII characters. When open the output XML file IE will report error.

Here is the first line of my XML file:
<?xml version="1.0" encoding="utf-8"?>

The problem is I don't know which non-ASCII chars are in these articles so I'm looking for a function which can encode all these non-ASCII chars into &#123; For example the double quote needs to be encoded into &#147;(left quote) and &#148;(right quote).

[coopster]

*bump*
You mean something like htmlentities() [php.net]?