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PHP calculate change

Posted on 2010-11-17
Last Modified: 2012-05-10
I have a class called payment where I get the amount that a person owes then the amount the person paid then the amount of change. I also need to provide the a member function to determine the largest amount of each denomination to return to the customer. This is really confusing me I am not sure do I make a variable for each denomination. and then the math to figure out how to distribute it.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<html xmlns="http://www.w3.org/1999/xhtml">
<title>Calculate Change</title>
<meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />


	class payment {

		public function __construct($paid,$owed) {
			$this->paid = $paid;
			$this->owed = $owed;

		public function getChange() {
			return $this->owed - $this->paid;

		public function getOwes() {
			return $this->owed - $this->paid;
		public function returnDenomination(){
			$one = 1.00;
			$five = 5.00;
			$ten = 10.00;
			$twenty = 20.00;
			$fifty = 50.00;
			$hundred = 100.00;
			$penny = 0.01;
			$nickle = 0.05;
			$dime = 0.10;
			$quarter = 0.25;
			if(this->paid === this->owed)
				echo "The customer is not owed any change. <br />"


	$payment = new payment($_POST["owed"],$_POST["paid"]);

	$change = $payment->getChange();
	$stillOwes = $payment->getOwes();

<h1>Calculate Change</h1>
<h1>Enter a transaction</h1>
<form action="CalculateChange.php" method="post">
<table border="0">
     <tr><td align="right">Amount Owed:</td>

         <td align="left">$<input type="text" size="6" name="owed"
              value="" /></td></tr>
     <tr><td align="right">Amount Paid:</td>
         <td align="left">$<input type="text" size="6" name="paid"
              value="" /></td></tr>
<tr><td colspan="2" align="center">
     <input type="reset" name="reset" value="Clear Form" /> &nbsp;
     <input type="submit" name="submit" value="Calculate Change" /></td></tr>


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Question by:rcham68
  • 2
LVL 31

Expert Comment

ID: 34157675
The simplest way to calculate how to generate change is to start with the largest currency denomination, and subtract as many of those as you can until the amount of change you need to give is less than the denomination, then move down to the next largest currency denomination, and repeat until you get to pennies.

E.g. if you have to give $13.57 back in change, the way you do it is:

Denomination          #          Total          New Subtotal
$50 bills         -        0          $0                     $13.57
$20 bills         -        0          $0                     $13.57
$10 bills         -        1          $10                 $ 3.57
$5   bills         -         0         $0                    $3.57
$1   bills         -         3         $3                    $0.57
25c coins      -          2         $0.50              $0.07
10c coins     -          0          $0                    $0.07
5c   coins      -         1           $0.05              $0.02
1c   coins      -         2           $0.02              $0.00

Total:         1x $10 bill,   3x $1 bill,   2x 25c coin,   1x 5c coin,    2x 2c coin

LVL 31

Accepted Solution

Frosty555 earned 500 total points
ID: 34157729

changetogive   =   13.57
twentybill = 0
tenbill = 0
fivebill = 0
onebill = 0
quarters = 0
dimes = 0
nickels = 0
pennies = 0

// 20 dollar bills
while changetogive >= 20 {
      changetogive -= 20
      twentybills += 1

// 10 dollar bills
while changetogive >= 10 {
      changetogive -= 10
      tenbills += 1

..... repeat for each type of currency ......

// nickels
while changetogive >= 0.05 {
      changetogive -= 0.05
      nickels += 1

// pennies
while changetogive >= 0.01 {
      changetogive -= 0.01
      pennies += 1

// finished!

total change to give is stored in the variables:

twentybill, tenbill,fivebill,
onebill, quarters, dimes,
nickels, pennies
LVL 19

Expert Comment

ID: 34157764
gee, the 50¢ piece never gets any respect.

Have it return an array where the key is the denomination and the value is the quantity.

I know I'd give my students a few extra points for such an answer.

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