Have you checked every other log file in your log folder/locations? From reading http://php.net/manual/en/function.syslog.php it seems that it might not go into php_error_log
.
Another point gathered in the documentation is:
On Windows NT, the syslog service is emulated using the Event Log.
I know it says NT
but it might have remained the same all this time. You might want to check the event viewer. I am on linux currently but I imagine its something like this:
Control Panel -> Administration -> Event Viewer
Edit: http://windows.microsoft.com/en-gb/windows-vista/open-event-viewer