Numbers and Math

PHP has exactly two numeric types: int for whole numbers and float for everything with a decimal point. You never declare which one you want — the operator and the operands decide.

The operators

<?php
$a = 17;
$b = 5;
echo $a + $b, "\n";
echo $a - $b, "\n";
echo $a * $b, "\n";
echo $a / $b, "\n";
echo intdiv($a, $b), "\n";
echo $a % $b, "\n";
echo $a ** $b, "\n";

//> 22
//> 12
//> 85
//> 3.4
//> 3
//> 2
//> 1419857

Why did division give me a decimal?

Because / is true division. It hands back an int only when the division comes out exact, and a float otherwise.

If you want the whole-number part, say so with intdiv(). If you want the leftover, use %. Coming from Python, note that PHP's / behaves like Python's /, and intdiv() fills in for Python's // — there is no // operator here.

What about negative numbers?

intdiv() chops toward zero. floor() chops toward negative infinity. They agree on positive numbers and disagree on negatives, which is a classic source of off-by-one bugs.

<?php
echo intdiv(-7, 2), "\n";      // toward zero
echo floor(-7 / 2), "\n";      // toward minus infinity
echo -7 % 3, "\n";             // the sign follows the LEFT operand
echo 7 % -3, "\n";

//> -3
//> -4
//> -1
//> 1

Integers do not wrap around

Overflow a PHP integer and it quietly becomes a float, instead of wrapping around to a negative number the way C would.

<?php
echo PHP_INT_MAX, "\n";
var_dump(PHP_INT_MAX + 1);

//> 9223372036854775807
//> float(9.2233720368548E+18)

You gain range and lose exactness: a float carries roughly 15–16 significant digits, and var_dump on this platform prints about 14 of them.

Floats are approximations

0.1 cannot be written exactly in binary, so neither can 0.1 + 0.2.

Compare floats by asking whether the gap between them is small enough: $\lvert a - b \rvert < 10^{-9}$. This is not a PHP quirk — it is how binary floating point works in every language.

The number toolbox

<?php
echo floor(2.7), " ", ceil(2.1), " ", round(2.5), " ", round(2.4567, 2), "\n";
echo number_format(1234567.891, 2), "\n";

//> 2 3 3 2.46
//> 1,234,567.89

Your exercise

Rectangle Area reads a width and a height, one per line, and prints their product.

The starter reads both values and casts them to int, so the answer is a single echo. The mistake the grader catches is decoration: Area: 15 and 15 sq units both fail against the expected 15. (Whitespace at the very end of your output is trimmed before the comparison, so a forgotten final line break is forgiven here — write it anyway, because a break in the middle of a multi-line answer is not.) Use * — not +, and definitely not /, which would drag a decimal point into your output. Multiplying two ints stays an int. The hidden case is a 1 by 999 rectangle, so nothing that quietly assumes a square will survive it.