precision - PHP Rounding Float

Question

I'm working on a system where I need to round down to the nearest penny financial payments. Naively I thought I would multiply up by 100, take the floor and then divide back down. However the following example is misbehaving:

echo 1298.34*100;

correctly shows:

129834

but

echo floor(1298.34*100);

unexpectedly shows:

129833

I get the same problem using intval for example.

I suspect the multiplication is falling foul of floating point rounding. But if I can't rely on multiplication, how can I do this? I always want to round down reliably, and I don't need to take negative amounts into consideration.

To be clear, I want any fractional penny amounts to be stripped off:

1298.345 should give 1298.34
1298.349 should give 1298.34
1298.342 should give 1298.34

question from:https://stackoverflow.com/questions/65844901/php-rounding-float

Answer 1

Since you mention you only use this for displaying purposes, you could take the amount, turn it into a string and truncate anything past the second decimal. A regular expression could do the job:

preg_match('/d+.{0,1}d{0,2}/', (string) $amount, $matches);

This expression works with any number of decimals (including zero). How it works in detail:

• d+ matches any number of digits
• .{0,1} matches 0 or 1 literal dot
• d{0,2} matches zero or two digits after the dot

You can run the following code to test it:

$amounts = [
    1298,
    1298.3,
    1298.34,
    1298.341,
    1298.349279745,
];
foreach ($amounts as $amount) {
    preg_match('/d+.{0,1}d{0,2}/', (string) $amount, $matches);
    var_dump($matches[0]);
}

Also available as a live test in this fiddle.