The Goal of this pattern is to have separate classes of logic in order to do condition composition without rewriting and maintaining the conditions at different places.

How to

All the classes should extend Kanel/Specification (abstract class)

$isAvailable = new isProductAvailable(); 
$isShippable = new isProductShippable();
$isOnSale = new isProductOnSale();

Then perform a composition of specification

$specification = $isAvailable->and($isOnSale)->xor($isShippable);

if ($specification->isSatisfiedBy($product)) {

}

Available operators :

and
or
xor
not

If you look for another implementation of the specification design pattern look at kanel/specifications2