permutations.php

<?php

function permute($items) {
	$permutes = [];
	permute_pairs($items, [], $permutes);
	return $permutes;
}

function permute_pairs($items, $parents = [], &$out = []) {
	for ($i = 0, $l = count($items); $i < $l; $i++) {
		$base = $items[$i];
		$subParents = $parents;
		$subParents[] = $base;
		if (count($items) === 1) {
			$out[] = $subParents;
		} else {
			$rest = $items;
			array_splice($rest, $i, 1);
			permute_pairs($rest, $subParents, $out);
		}
	}
}


var_dump([
	permute(['a', 'b', 'c', 'd']),
]);