The sanitize checks only if all characters used in a given input can be part of an integer, float, etc. If you like to have a strict validation, use FILTER_VALIDATE_INT (or other validating filters).

> sanitize checks only if all characters used in a given
> input can be part of an integer, float, etc

Where does it say that? http://php.net/manual/en/intro.filter.php
From what I could gather from the docs, 
filter_var should return:
 Returns the filtered data, or FALSE if the filter fails. 
and filter_input should return:

Value of the requested variable on success, FALSE if the filter fails, or NULL if the variable_name  variable is not set. If the flag FILTER_NULL_ON_FAILURE is used, it returns FALSE if the variable is not set and NULL if the filter fails.

I understand that FILTER_VALIDATE_INT is not as stringent as FILTER_SANITIZE_NUMBER_INT, but given these input and return combinations:

input   return
x.9      9
x-9     -9
x       ''
-       -
+       +
++      ++
--      --

Then given the string, "<!-- test -->", how is a return value of "----" a valid INT with a minimum of 1?

I assume a regular expression is behind this filter, and at the very least I expect that it needs to be updated to only match no more than one "-" or "+" only if it is followed by a "." or [0-9].  Possibly others depending on localization settings.

It's not much of a validation filter as it exists currently.

I used FILTER_SANITIZE_NUMBER_INT and FILTER_VALIDATE_INT backward in my last comment.

Still, neither filter should ever return '----' as an INT with a minimum value of 1.

This result is expected, the documentation for FILTER_SANITIZE_NUMBER_INT says: "Remove all characters except digits, plus and minus sign".

FILTER_VALIDATE_INT works as you expect: It does not outputs invalid numbers and does range check using the min/max_range options.

Still FILTER_SANITIZE_NUMBER_INT function return Minus(-) Sign