Doctrine Collections is a library that contains classes for working with arrays of data. Doctrine Collections provides an interface named Doctrine\Common\Collections\Collection that resembles the nature of a regular PHP array. That is, it is essentially an ordered map that can also be used like a list. A Collection has an internal iterator just like a PHP array. In addition, a Collection can be iterated with external iterators, which is preferable. To use an external iterator simply use...