I found that when I launch eclipse.exe, and if I try to debug right away
(using PDT+XDebug) that it doesn't default to using the "Internal Web
Browser".
"Internal Web Browser" seems available only if I do "Window->Web Browser"
before launching my first debug session. If I do that, then I'm able to
use it. For the detailed steps that I am performing, see
http://robsnotebook.com/php_debugger_pdt_xdebug, step 8.a.ii.v.ii.