Group by operation implemented in PHP on arrays.

array_group_by.php

<?php
function array_group_by (array $array, callable $mapping) 
{
	$buckets = [];
	foreach ($array as $item) {
		$key = $mapping($item);
		if (!isset($buckets[$key])) {
			$buckets[$key] = [$item];
		} else {
			$buckets[$key][] = $item;
		}
	}
	return $buckets;
}