The AT&T framework was an R&D framework designed by AT&T a few years ago and then released to Apache. According to github, it is still active. I am not aware of any commercial use of the product (though I suspect AT&T use it themselves).
The three main frameworks used out there are:
- WSO2 Balana. This is an open-source Java PDP. It is also bundled as part of WSO2 Identity Server. Most open source users use Balana.
- SunXACML: this is the original XACML 3.0 implementation. It is pretty old but sturdy (it's been around for 10 years or so). It is also open-source and Java.
- Axiomatics Policy Server. This is a commercial solution (disclaimer: I work for Axiomatics). It is the most prevalent commercial solution out there. It is also implemented in Java but supports integration for PHP too.
You can find this information on XACML's wikipedia page. There is another interesting engine called AuthZForce but I have little experience with it.
All these engines, AFAIK, use XACML's XML format to store policies. You claim you need JSON. There is no Policy profile of XACML in JSON. There was a question on the topic which you can read here.
Given The Axiomatics Policy Server is exposed as a JSON API, you can use it to integrate with PHP. Other engines probably have a similar approach. If you use Amazon AWS, you can request a copy of the Axiomatics Policy Server AMI.
ABAC is technology-neutral meaning it is not specific to Java, Ruby, .NET, PHP or any other language. What the PDP engine is written in is irrelevant to what your application is written in so long as you can integrate the two together.
I hope this helps,
David.