Getting the contextid of a user through Web Services API

by Doug Stevens.  

Hi all, 

I'm writing a php script to integrate our SIS with Moodle.  As part of this I need to be able to lookup the contextid for a user. I can do this with a direct database call of "select contextid from mdl_context where instanceid=....", however I can't seem to find an API method that does this same thing. Anyone know a method that will do this?

Thanks

Doug