Numbers and Math

PHP handles everyday arithmetic with the operators you know, plus a large library of math functions. This lesson covers the arithmetic operators and their quirks — especially division — rounding, formatting numbers for display, random numbers, and how to handle money without floating-point errors.

Arithmetic operators

PHP 
<?php

$a = 17;
$b = 5;

echo $a + $b, "\n";
echo $a - $b, "\n";
echo $a * $b, "\n";
echo $a / $b, "\n";     // division
echo $a % $b, "\n";     // remainder (modulo)
echo $a ** 2, "\n";     // exponentiation
echo -$a, "\n";
Output
22
12
85
3.4
2
289
-17

Division

Unlike many languages, / in PHP doesn't throw away the fraction: dividing two integers gives a float unless the result is a whole number. For integer division, use intdiv():

PHP 
<?php

var_dump(10 / 2);
var_dump(7 / 2);
var_dump(intdiv(7, 2));
var_dump(-7 % 2);          // remainder keeps the sign of the left side
var_dump(fmod(7.5, 2));    // remainder for floats
Output
int(5)
float(3.5)
int(3)
int(-1)
float(1.5)

Dividing by zero throws a DivisionByZeroError, which stops the script unless it's caught (lesson 44):

PHP 
<?php

$items = 10;
$boxes = 0;
echo $items / $boxes;
Output
Fatal error: Uncaught DivisionByZeroError: Division by zero in main.php:5
Stack trace:
#0 {main}
  thrown in main.php on line 5

Rounding

PHP 
<?php

echo round(3.14159, 2), "\n";
echo round(2.5), " ", round(3.5), " ", round(-2.5), "\n";   // halves round away from zero
echo round(1234.5678, -2), "\n";                           // to the nearest hundred
echo floor(4.7), " ", ceil(4.2), "\n";
echo round(2.5, 0, PHP_ROUND_HALF_EVEN), "\n";             // banker's rounding
Output
3.14
3 4 -3
1200
4 5
2

PHP's round() rounds halves away from zero by default — the school rule. Pass a mode such as PHP_ROUND_HALF_EVEN (or the newer RoundingMode enum) for other behaviour.

Math functions

PHP 
<?php

echo abs(-7.5), "\n";
echo sqrt(144), "\n";
echo pow(2, 10), "\n";
echo max(3, 9, 4), " ", min([3, 9, 4]), "\n";
echo M_PI, "\n";
echo round(pi() * 2 ** 2, 3), "\n";
echo intdiv(PHP_INT_MAX, 1_000_000_000), "\n";
echo log(M_E), " ", log10(1000), "\n";
Output
7.5
12
1024
9 3
3.1415926535898
12.566
9223372036
1 3

max and min accept either several arguments or one array.

Formatting numbers

number_format rounds a number and adds thousands separators — ideal for display:

PHP 
<?php

$revenue = 1234567.891;

echo number_format($revenue), "\n";
echo number_format($revenue, 2), "\n";
echo number_format($revenue, 2, ',', '.'), "\n";   // European style
echo number_format(0.5), "\n";
Output
1,234,568
1,234,567.89
1.234.567,89
1

Lesson 18 shows printf/sprintf for more control over layout.

Money: avoid floats

Because floats can't represent most decimal fractions exactly, small errors creep into money calculations:

PHP 
<?php

$total = 0;
foreach ([0.10, 0.20, 0.30] as $price) {
    $total += $price;
}
var_dump($total);
var_dump($total == 0.6);
Output
float(0.6000000000000001)
bool(false)

Two reliable approaches:

  • Store whole cents as integers and only format them as currency for display.
  • Use decimal arithmetic — the bcmath extension's BcMath\Number class (PHP 8.4+) does exact decimal maths with normal operators.
PHP 
<?php

$cents = 0;
foreach ([10, 20, 30] as $priceInCents) {
    $cents += $priceInCents;
}
printf("Total: %d.%02d\n", intdiv($cents, 100), $cents % 100);

$a = new BcMath\Number('0.10');
$sum = $a + '0.20' + '0.30';
echo "Exact: $sum\n";
var_dump($sum == new BcMath\Number('0.6'));
Output
Total: 0.60
Exact: 0.60
bool(true)

Random numbers

random_int($min, $max) returns a random integer between the limits, inclusive. It uses a cryptographically secure source, so it's safe for things like generating codes:

PHP 
<?php

$dice = random_int(1, 6);
echo $dice >= 1 && $dice <= 6 ? "Rolled a valid die\n" : "Bug!\n";

$rng = new Random\Randomizer(new Random\Engine\Mt19937(42));   // seeded: repeatable
echo $rng->getInt(1, 100), " ", $rng->getInt(1, 100), "\n";
echo $rng->shuffleArray(['a', 'b', 'c', 'd']) == ['a', 'b', 'c', 'd'] ? "same order\n" : "shuffled\n";
Output
Rolled a valid die
43 68
shuffled

The Random\Randomizer class (PHP 8.2+) offers shuffling, picking array keys and random strings. With a seeded engine it produces the same sequence every run — useful for tests and simulations.

Try it yourself

Split a restaurant bill between friends, working in cents to avoid rounding errors.

Try it yourself · main.php
Edit the code, copy it into main.php, then run it with php main.php (PHP 8.5). How to install PHP
Output
Total with tip: 100.51
Each pays 33.50, plus 1 cent(s) left over
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