Subdomains

I have a site that runs CakePHP (accessed via www.[domain].com). I now
want to install Mantis bug tracker on a subdomain on the same site. So
if I type mantisbt.[domain].com it takes me to Mantis rather than to
the normal web root - and therefore does not fire up the Cake site.
What settings do I need to change to make this happen - if it is
possible? Or is there a better way of doing this?.