IIS not displaying detailed PHP errors - shows server fault 500 instead

How about logging them instead?

Edit php.ini and set

error_reporting = E_ALL & ~E_NOTICE
log_errors = On

And either

error_log = syslog to log to the Windows Event log, or

error_log = location/of/logfile to log to a file.

Assuming you have all the right settings in your php.ini file

1. Select your website in IIS Manager
2. On the right, double click Error Pages icon
3. On far right under 'Actions', click 'Edit Feature Settings' link
4. On pop up that appears, choose 'Detailed errors' radio button
5. Refresh your PHP page to see detailed errors

Thanks to last comment on this page