R1:a5903d2a53bb

Use head_key() and last_key() to explicitly communicate intent

Summary:
PHP arrays have an internal "current position" marker. (I think because foreach() wasn't introduced until PHP 4 and there was no way to get rid of it by then?)

A few functions affect the position of the marker, like reset(), end(), each(), next(), and prev(). A few functions read the position of the marker, like each(), next(), prev(), current() and key().

For the most part, no one uses any of this because foreach() is vastly easier…