lesson - do not use floats for money

floats.php

<?php

#don't use floats for money!

$a = 110;            #   $110
$b = 1.10 * 100;     #   100 items at $1.10 each
#$b = 110 -1E-13;

print "a: "; var_dump($a);
print "b: "; var_dump($b);

print "\n";

print "a: $a\n";
print "b: $b\n";

print "\n";

if ($a < $b) {
  print "a is less than b\n";
}

if ($b < $a) {
  print "b is less than a\n";
}

print "difference: " . ($a - $b)."\n";

Output:

a: int(110)
b: float(110)

a: 110
b: 110

a is less than b
difference: -1.4210854715202E-14

That's a very small difference. It will hardly make any trouble as long as you know that you shouldn't do direct comparisions like value1 == value2 but instead use abs(value1 - value2) < 0.0001.

True, I could have written "do not use floats for money unless the code is designed to handle it".

Personally, for storing monetary values, I still prefer using fixed point types or faking them with ints. I think the occasional $value/100 when outputting is a small price to pay to be able to use direct comparisons.

I agree that decimals are better to use. In MySQL there's a DECIMAL type (although at work we use integers with the two least significant digits being öre/cents/...), but in PHP you either have to create your own class via integers or use a library like BC or GMP.

serialize will show the detail that var_dump, var_export and print all hide:
$ php -r "print serialize(1.1 * 100);"
d:110.0000000000000142108547152020037174224853515625;