Floats

A float (floating-point number) is a number with a decimal part.

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<?php
$price = 19.99;
$pi = 3.14159;
$small = 0.5;

var_dump($price, $pi, $small);

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float(19.99)
float(3.14159)
float(0.5)

Scientific notation

Very large or very small numbers can be written with e.

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<?php
var_dump(1.5e3);   // 1.5 * 10^3
var_dump(2e-3);    // 2 * 10^-3

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float(1500)
float(0.002)

Floats are approximate

Computers store floats in binary, and many decimal values cannot be stored exactly. This is true in every programming language, not only PHP.

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<?php
var_dump(0.1 + 0.2);
var_dump(0.1 + 0.2 == 0.3);

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float(0.30000000000000004)
bool(false)

So never compare floats with ==. Compare with a small tolerance instead.

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<?php
$a = 0.1 + 0.2;
$b = 0.3;

var_dump(abs($a - $b) < 0.00001);

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bool(true)

Floating-Point Pitfalls goes further, including how to handle money.

Rounding for display

round() and number_format() are made for showing floats to people.

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<?php
$price = 19.987;

echo round($price, 2) . "\n";
echo number_format($price, 2);

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19.99
19.99

Special float values

PHP also has INF (infinity) and NAN (not a number). Check for them with is_infinite() and is_nan().