<?php

error_reporting(-1);

$amount = 54500;

$bills = array(
	100    =>    23,
	500    =>    5,
	1000   =>    0,
	5000   =>    200
);
$bills = array_reverse($bills);
$array = array();
foreach ($bills as $nominal => $value) {
		$countBanknote = floor($amount / $nominal);
                  if($amount >= $nominal and $countBanknote < $value) {
	     	$value = $value - $countBanknote;
	     	$amount = $amount % $nominal;
	     	$return = "{$nominal} x {$countBanknote}";
	     	$array[] = $return;
	     	var_dump($bills);
	   }
   }
$array = implode(" ", $array);
echo $array;