A common implementation for the criterion object is the Specification pattern. A specification is a simple predicate that takes a Domain object and returns a boolean. Given a Domain object, it will return true if it specifies the specification, and false otherwise:

interface PostSpecification
{
    /**
     * @return boolean
     */
    public function specifies(Post $aPost);
}

We just need to add a query method to our Repository:

interface PostRepository
{
    // ...
    public function query($specification);
}