In Moodle 1.8, setting debug to DEVELOPER meant that pages were served as XHMTL strict and application/xhtml+xml, which is very good for finding invalid HTML, but breaks some legacy javascript. In Moodle 1.9, debug level, and whether to serve as XHTML are separate settings.
Anyway, you should be able to use debug 'ALL' safely. (Or hack print_header to stop it setting the mime type to application/xhtml+xml in developer debug mode.)
Web services
Firing a client to an external service in moodle
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