error 5 the response is not a valid json string

[macnux]

Hi
im using dremaweaver mx and php 5.2.5 and mx ajax toolbox from interaktonline
i want to make simple ajax web service
and this is a tutorial that i followed
http://www.interaktonline.com/Docume...k_username.htm
it gives me error "error 5 the response is not a valid json string"
even very simple services gives me this error !!!!
any help?

thanks in advance.