PHP Errors not showing up in event viewer

The appropriate php.ini settings are set:

log_errors = On

error_log = syslog

However, errors are not being logged in Event Viewer -> Windows Logs -> Application.

I tried a few different things. If I run the php script containing the error from the command line then the error is written to the Event Viewer, no problem. If I substitute the error_log setting with a path like C:/temp/error.log then IIS will write the error to the file.

How do I make error_log = syslog work so that IIS logs PHP errors in the event viewer? I'm hoping it's a simple switch or button I'm missing.

Version Info: IIS 7.5 and PHP 5.5.