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

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http://serverfault.com – I'm used to serving sites from Apache, but am now forced to use an IIS server. A number of my PHP pages are returning error 500 and I can't debug because IIS is suppressing the error messages. I've tried ini_set('display_errors',1); in code but to no avail. Errors are shown on some sites but not the one in particular that I want to debug so I don't think it is a global PHP setting, but rather something to do with the site settings for that site. I can't find a clear answer as to how to enable displaying PHP errors. I've found this link but am not sure where to find "system.webServer->ht (HowTos)