Joshua Eichorn has found a tutorial someone else has written on creating a stock lookup plugin for WordPress with his HTML_AJAX package.

The only problem I see is the code won't work in PHP5 since introspection will return the method name as GetQuote instead of getquote.

He gives the code changes you'll need to make to get it all working in your PHP5 installation, and includes another option as well - registering all of the functions you want to use instead of just automagically using them.