Slightly more approachable intros to this topic:
Hello, declarative world: https://codon.com/hello-declarative-world
Reasoned PHP: https://igor.io/2014/08/06/reasoned-php.html
Hello, declarative world: https://codon.com/hello-declarative-world
Reasoned PHP: https://igor.io/2014/08/06/reasoned-php.html
I haven't had the chance to build anything that uses them. But having become familiar, I think there are lots of potential applications. I always thought that Prolog was strange. Logic programming makes a lot more sense as a library IMO.