Wrong PHP string equality

Question

For some reason, PHP decided that if:

$a = "3.14159265358979326666666666"

$b = "3.14159265358979323846264338"

$a == $b is true.

Why is that, and how can I fix that?

It ruins my code.

Answer 1

The Problem

PHP converts strings (if possible) to numbers (source). Floating points have a limited precision (source). So $a == $b because of rounding errors.

The Fix

Use === or !==.

Try it

<?php

$a = "3.14159265358979326666666666";
$b = "3.14159265358979323846264338";

if ($a == $b) {
    echo "'$a' and '$b' are equal with ==.<br/>";
} else {
    echo "'$a' and '$b' are NOT equal with ==.<br/>";
}

if ($a === $b) {
    echo "'$a' and '$b' are equal with ===.<br/>";
} else {
    echo "'$a' and '$b' are NOT equal with ===.<br/>";
}
?>

Results in

'3.14159265358979326666666666' and '3.14159265358979323846264338' are equal with ==.
'3.14159265358979326666666666' and '3.14159265358979323846264338' are NOT equal with ===.

Note

When you want to do high precision mathematics, you should take a look at BC Math.

Answer 2

You could use === in the equality test.

$a = "3.14159265358979326666666666";
$b = "3.14159265358979323846264338";

if($a===$b)
{
    echo "ok";
}
else
{
    echo "nope";
}

This code will echo nope.

Comparing with == is a loose comparison, and both strings will be converted to numbers, and not compared right away.

Using === will perform a string comparison, without type conversion, and will give you the wanted result.

You can find more explanations in the PHP manual:

• PHP type comparison tables

• Comparison Operators