I thought I'd share a PHP library implementing this idea by Evans and Fowler. Written for PHP5, it's a library for defining custom specifications using composition and inheritance.
He gives the example of "Person" objects with name and age properties. A search might involve looking directly at the properties to location one that's, say, younger than 35 and has a last name of Johnson. His specification class replaces this and makes it more reusable by applying things like GreaterOrEqualSpecifications and EqualSpecifications to standardize the evaluation. Then its just a quick call to the isSatisfiedBy method to check for correctness.