What is the php_binary serialization handler?

When I do phpinfo() or php -i from the CLI, I get the following output:

session

Registered save handlers => files user sqlite memcached
Registered serializer handlers => php php_binary wddx

I was not aware that PHP serialization supported a binary format, and there does not appear to be anything in the documentation about it.

I was going to use igbinary for session serialization to a memcached server, so I'm wondering how php_binary compares.