PHP money string conversion to integer error

Question

I have a small financial application with PHP as the front end and MySQL as the back end. I have ancient prejudices, and I store money values in MySQL as an integer of cents

Answer 1

If you want precision, you should store your money values using the DECIMAL data type in MySQL.

Answer 2

Currency/money values should never be stored in a database (or used in a program) as floats.

Your integer method is fine, as is using a DECIMAL, NUMERIC or MONEY type where available.

Your problem is caused by $dollars being treated as a float and PHP doesn't have a better type to deal with money. Depending on when $dollars is being assigned, it could be being treated as a string or a float, but is certainly converted to a float if it's still a string for the * 100 operation if it looks like a float.

You might be better off parsing the string to an integer "money" value yourself (using a regex) instead of relying on the implicit conversions which PHP is doing.

Answer 3

Your "prejudices" about floats will never be overcome - it's fundamental to the way they work. Without going into too much detail, they store a number based on powers of two and since not all decimal number can be presented this way, it doesn't always work. Your only reliable solution is to store the number as a sequence of digits and the location of the decimal point (as per DECIMAL type mentioned above).

I'm not 100% on the PHP, but is it possible the multiplication is converting the ints to floats and hence introducing exactly the problem you're trying to avoid?

Answer 4

You should never ever store currency in floating point, because it always get results you don't expect.

Check out php BC Maths, it allow you to store your currency as string, then perform very high precision arithmetic on them.

Answer 5

Instead of using

$cents = (integer) ($dollars * 100);

you may want to try to use:

$cents = bcmul($dollars, 100, 2);

Answer 6

There's no point in storing money as integer if you enter it through a floating point operation (no pun intended). If you want to convert from string to int and be consistent with your "prejudice" you can simply use string functions.

You can use an arbitrary precision library to divide by 10 (they handle numbers internally as strings), e.g. bcdiv() or gmp_div_q(), but of course, you could have also used it from the beginning for all the math.

Or you can use plain string functions:

<?php
// Quick ugly code not fully tested
$input = '156.64';
$output = NULL;

if( preg_match('/\d+(\.\d+)?/', $input) ){
    $tmp = explode('.', $input);
    switch( count($tmp) ){
        case 1:
            $output = $tmp[0];
            break;

        case 2:
            $output = $tmp[0] . substr($tmp[1], 0, 2);
            break;

        default:
            echo "Invalid decimal\n";
    }
}else{
    echo "Invalid number\n";
}

var_dump($output);

?>