filter_input with 0 returns false

Question

I'm using filter_input now and I want it to return true on every number (0-9) and false on non-numbers (a-z, A-Z etc..)

Now, I use this script (just an example script but the idea is the same):

<?php

$i = filter_input(INPUT_GET, 'i', FILTER_SANITIZE_NUMBER_INT);

if ($i)
{
    echo 'good';
}
else
{
    echo 'bad';
}

If I use 1-9 as GET variable (so like test.php?i=1), it shows good. If I use a-Z as GET variable (so like test.php?i=a), it shows bad. So far so good.

But if I use 0 as GET variable, it shows bad as well. The variable can be 0 and should be true when the GET variable is 0.

Can anybody explain me why 0 returns false? I mean the GET variable is set and 0 is a number. Is it just because 0 is a falsey value?

Answer 1

0 is a falsey value in PHP. if (0) is false.

Further, you're using the sanitisation filter, which will basically never fail. You want to use FILTER_VALIDATE_INT to validate whether the input is an integer.

Further, filter_input returns either false, null or 0 here. They're all equally falsey, meaning == false. You need to explicitly distinguish between them:

$i = filter_input(INPUT_GET, 'i', FILTER_VALIDATE_INT);

if ($i === null) {
    echo 'value not provided';
} else if ($i === false) {
    echo 'invalid value';
} else {
    echo 'good';
}