Simple addition and subtraction not working correctly - php

Question

I've inherited a project written in PHP which has a double entry style cashbook, which requires that the amount going in is equal to the amount going out.
When validating figures entered via a form (to work out if it equals zero), it runs the code below which works fine until there is a decimal number, which seems to make it randomly fall over and return an incorrect value.
In the example below, it returns a really tiny number from 53.87 - 53.87, which should be zero.This has been stripped back to eliminate other things from causing problems, so I've removed a lot of input validation etc.
<?php $inputs = array(
array(
"in" => '',
"out" => '249.6',
),
array(
"in" => '',
"out" => '396',
),
array(
"in" => '554.4',
"out" => ''
),
array(
"in" => '145.07',
"out" => ''
),
array(
"in" => '',
"out" => '53.87',
),
);
$fTotal = 0;
echo "Start at 0: ";
foreach($inputs as $key=>$sRef) {
$itemValid = true;
$aItem = array();
$amountIn = $inputs[$key]['in'];
$amountOut = $inputs[$key]['out'];
if ($itemValid) {
echo "'".$fTotal."'";
$is_in = 0;
if ($amountIn > 0.0) {
$is_in = 1;
echo "+";
$amount = $amountIn;
$fTotal = $fTotal + $amountIn;
} else {
$is_in = 0;
echo "-";
$amount = $amountOut;
$fTotal = $fTotal - $amountOut;
}
echo "'".$amount."'=";
echo "'".$fTotal."' | ";
$aItem["is_in"] = $is_in;
$aItem["amount"] = $amount;
$aItems[] = $aItem;
}
}
You can run this on a sandbox here.
Here is the expected output:
Start at 0: '0'-'249.6'='-249.6' | '-249.6'-'396'='-645.6' | '-645.6'+'554.4'='-91.2' | '-91.2'+'145.07'='53.87' | '53.87'-'53.87'='0' |
Here is the actual output:
Start at 0: '0'-'249.6'='-249.6' | '-249.6'-'396'='-645.6' | '-645.6'+'554.4'='-91.2' | '-91.2'+'145.07'='53.87' | '53.87'-'53.87'='-4.9737991503207E-14' |
What is wrong here?
Update
Following help below, here's the working code for anyone who stumbles on this in the future.

Answer 1

This is happening because of floating point (decimal) maths where computers like binary maths. Sometimes decimal numbers don't have a nice representation in binary so these tiny differences happen.
If you can state that every number can be rounded to 2 decimal points, and based on your limited dataset, wrapping your 'sums' in number_format($fTotal + $amountIn , 2) etc can sort this out for you.
Alternatively for a "more accurate", just wrap the final one eg. echo "'".number_format($fTotal, 2)."' | "; (or 0 or whatever)

Answer 2

Your numbers are being interpreted as strings.
Try this:
<?php
$inputs = array(
array(
"in" => 0,
"out" => 249.6,
),
array(
"in" => 0,
"out" => 396,
),
array(
"in" => 554.4,
"out" => 0
),
array(
"in" => 145.07,
"out" => 0
),
array(
"in" => 0,
"out" => 53.87,
),
);
$fTotal = 0;
echo "Start at 0: ";
foreach($inputs as $key => $sRef) {
$itemValid = true;
$amountIn = $sRef['in'];
$amountOut = $sRef['out'];
if ($itemValid) {
echo $fTotal;
$is_in = 0;
if ($amountIn > 0.0) {
$is_in = 1;
echo "+";
$amount = floatval($amountIn);
$fTotal = floatval($fTotal) + floatval($amountIn);
} else {
$is_in = 0;
echo "-";
$amount = floatval($amountOut);
$fTotal = floatval($fTotal) - floatval($amountIn);
}
echo $amount;
echo ' = ' . $fTotal . " | ";
}
}

Answer 3

The bcsub() function in PHP is an inbuilt function and is used to subtract one arbitrary precision number from another.
Syntax:
string bcsub ( $num_str1, $num_str2, $scaleVal)
Example :
<?php $num_str1 = "8"; $num_str2 = "3"; $res = bcsub($num_str1, $num_str2) echo $res; ?>
Result:
5