Undefined index: SSL_CLIENT_CERT

While working with a PHP application running on Apache that uses client certificates, I ran into the following error:

Undefined index: SSL_CLIENT_CERT

The fix was simple.  Make sure you have the following in your apache ssl configuration file:

<VirtualHost _default_:443>
...

SSLOptions +FakeBasicAuth +ExportCertData +StrictRequire

...
</VirtualHost>

It might be commented out.