$pdf = new Prince('C:\Program Files\Prince\Engine\bin\prince.exe');
$pdf->convert2($tmpfile, 'tmp/out.pdf');
The 'tmp/out.pdf' path looks a bit suspicious; I think that it might be better to specify an absolute path for the PDF output to ensure that it is written to the correct directory.
$ prince C:\Dir01\Dir02\test.html C:\Dir01\Dir02\out.pdf
it fails with this error:
prince: warning: failed to load external entity "file:///C%3A/Program"
prince: C:\Program: error: could not load input file
Running Prince from the command-line with a path that contains spaces is a bit awkward, as "prince C:\Program Files\Blah" looks like two arguments not one: the first is "C:\Program" and the second is "Files\Blah"! Try wrapping the argument in quotes or using "Progra~1" instead of "Program Files".