Takes a sum of money, and returns an array expressing this sum in terms of a currency's denominations in the most efficient way possible.

getChange.php

<?php

// This function takes an array of denominations for a currency and a sum, and returns an array describing the most efficient way to express the sum in terms of denominations.
// E.g. A sum, such as 200 EUR can be expressed in terms of 100x2, which is inefficient, as there is the denomination 200 for this job.
// The function will return the most efficient way to express the sum, not 100x2, but 200x1.
// Recommended to round both, the expected and actual value to 2 decimal places.

// FUNCTION
// getChange function
function getChange($sum, $denominations) {
    // Initiate empty results array
    $results = [];


    foreach ($denominations as $denomination) {
        $frequency = (int)(floor(($sum) / $denomination));
        $results[$denomination] = $frequency;
        $sum = round($sum - ($denomination * $frequency), 2);
    }

    return $results;
}
// END FUNCTION


// Currency denominations for sample use and test
$denominations = ['500', '200', '100', '50', '20', '10', '5', '2', '1', '0.50', '0.20', '0.10', '0.05', '0.02', '0.01'];


// BEGIN TEST
// Error counter
$errors = 0;

// Number of iterations
$i = 888.88;

// Expected value is 0.00 from the start
$val_exp = 0.00;
while ($val_exp <= $i) {
    // Call getChange function
    $results = getChange($val_exp, $denominations);

    // Reset calculated value;
    $val_is = 0;

    // Go through each denomination, add value to calculated total
    foreach ($results as $denomination => $frequency) {
        if ($frequency) {
            $val_is += ($denomination * $frequency);
        }
    }

    // Round both values, expected and actual, to 2 d.p.
    $val_exp = round($val_exp, 2);
    $val_is = round($val_is, 2);

    // Increase error counter if difference detected
    if ($val_is != $val_exp) {
        $errors += 1;
    }

    // Increase expected value by 0.01
    $val_exp += 0.01;
}

// Echo number of errors encountered
echo "$errors errors encountered for $i iterations<br><br>";
// END TEST


// SAMPLE USE
$sum = 372.37;
$results = getChange($sum, $denominations);

echo "Best way to return change for $sum:<br><br>";
foreach ($results as $denomination => $frequency) {
    if ($frequency) {
        echo "$frequency times " . round($denomination, 2) . " EUR<br>";
    }
}
// END SAMPLE USE