php string to int issue

I got a strange issue on PHP, i'm trying to cast a string to integer and got as result an empty int variable...

The variable is originally a string extracted from Visual Composer template variable.

<?php

var_dump($sec_id);
$sec_id = (int)$sec_id;
var_dump($sec_id);

?>

Here is the output:

Do I forget something?

using var_dump(unpack('C*', $sec_id)); got the real output of the passed string, the issue is in the template variable creation.