Just a very simple demo showing how to get a filtered subset from a collection by using generators.

FilteredSubsetWithGeneratorsDemo.php

<?php

/**
 * PHP 5.5+ Required
 * How to get a filtered subset from a collection by using generators.
 * @author http://www.github.com/rmruano
 */

/* 1. Classes definition ------------------------------------------------------------------------ */

    class MyUser {
        /**
         * @var string
         */
        private $name;

        /**
         * @var $age
         */
        private $age;

        /**
         * @param string $name
         * @param int|null $age
         */
        public function __construct($name, $age = null) {
            $this->name = $name;
            $this->age = $age;
        }

        /**
         * @return string
         */
        public function getName() {
            return $this->name;
        }

        /**
         * @return int|null
         */
        public function getAge() {
            return $this->age;
        }
    }

    class MyGroup {
        /**
         * @var MyUser[]
         */
        private $objects;

        /**
         * @param MyUser $user
         * @return $this
         */
        public function add(MyUser $user) {
            $this->objects[] = $user;
            return $this;
        }

        /**
         * Get objects matching the filter
         * @see http://php.net/manual/en/language.generators.overview.php
         * @param int $minAge
         * @param int $maxAge
         * @return MyUser[]
         */
        public function ageRange($minAge=0, $maxAge=PHP_INT_MAX) {
            foreach ($this->objects as $user) {
                $age = $user->getAge();
                if ($age!=null && $age>=$minAge && $age<=$maxAge) {
                    yield $user;
                }
            }
        }
    }

/* 2. Demo usage ------------------------------------------------------------------------ */

    $myGroup = (new MyGroup())
        ->add(new MyUser("David",35))
        ->add(new MyUser("Phil",20))
        ->add(new MyUser("Bruce",39));

    foreach($myGroup->ageRange(30,40) as $myObject) {
        echo $myObject->getName() . " (Age ".$myObject->getAge().")".PHP_EOL;
    }

/*
  EXPECTED OUTPUT:
       David (Age 35)
       Bruce (Age 39)
*/