why does filter_input() return NULL when I manually assign value to $_POST array?

Question

I'm following some exercises about data validation and decided to mess around with the filter_input() function. When I try to run this code:

$_POST['var'] = 10;

$filtered = filter_input(INPUT_POST, 'var', FILTER_VALIDATE_FLOAT);

var_dump($filtered);

var_dump($filtered) returns NULL. I know the code works if $_POST['var'] is assigned a value by submitting a form, but I'm just wondering why manually assigning a value to the $_POST[] array doesn't return float(10)?

Answer 1

filter_input() doesn't read from the _POST/_GET/_COOKIE/_SERVER/_ENV superglobal arrays. It reads from the original sources of those values respectively (which are also used to populate the superglobals).

Answer 2

This is because INPUT_POST works on the original POST values, modifications to them in your script have no effect. However, if you want to filter that var you can use:

filter_var($_POST['var'], FILTER_VALIDATE_FLOAT);

Answer 3

If you want to set a default input value and filter, use filter_var. Because filter_input doesn't filter current values of superglobals.