This is a bit of a pain to enable, but you need to follow these steps to do it:
IIS PHP Manager
Open IIS Manager, click on the server node, then click on “PHP Manager” assuming you have installed PHP using the Web Platform Installer.
Now click on “Configure Error Reporting” link, set the type of PHP Error Reporting: “Development Machine” and set the error log file to valid location, the default is normally good enough.
IIS Configuration Editor
errorMode – “DetailedLocalOnly” Change To: “Detailed”
existingResponse – “Auto” Change To: “PassThrough”
display_errors = On
log_errors = On
error_reporting = E_ALL | E_STRICT
Then restart IIS to commit the changes.
In your browser you may need to untick “Show friendly HTTP Error messages” under the Internet Options->Advanced tab.
Tracing and Troubleshooting Failed Requests Using Tracing in IIS 7