Setting up DGB PHP debugging against XAMPP

Here's one in case anyone is pulling out their hair trying to work out
why they get the following message in their apache error.log:

PHP Warning: Module 'Zend Optimizer' already loaded in Unknown on line 0
PHP Warning: Zend Optimizer: module registration failed! in Unknown on
line 0

It seems that dbg somehow conflicts with Zend in a XAMPP installation,
and needs to be installed as a Zend extension. Here are the instructions:

http://rilnet.org/en/node/14

Don't believe installation instructions