Collection macro to peek backwards at the last value in a map.

collection-peekMap.php

Collection::macro('peekMap', function (callable $callback) {
    $lastValue = null;

    return $this->map(function ($value, $key) use ($callback, $lastValue) {
        $newValue = $callback($value, $key, $lastValue);
        $lastValue = $newValue;

        return $newValue;
    });
});