<?php
 
error_reporting(-1);
 
$bills = [500, 100];
$totalBills = [5, 5, 5, 5];
$rndCombination = [0, 0, 1, 5];
 
print_r($rndCombination);
 
function nextCombination($total, $combination){
	if($total == $combination){
		return 'Максимальная комбинация.';
	}
	$N = count($total) - 1;
	for($N; $N>0; $N--){
		if($combination[$N] < $total[$N]){
			$combination[$N] += 1;
			break;
		}
		else{
			$combination[$N] = 0;
		}
	}
	return $combination;
}
 
print_r(nextCombination($totalBills, $rndCombination));

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