Strips all chars which are not 0-9, + and -.
<?php
$flags = array();
$filter = FILTER_SANITIZE_NUMBER_INT;
include 'presentations/slides/input-filter/render-form.php';
include 'presentations/slides/input-filter/check-data.php';
?>
Output
<?php
$flags = array();
$filter = FILTER_SANITIZE_NUMBER_INT;
include 'presentations/slides/input-filter/render-form.php';
include 'presentations/slides/input-filter/check-data.php';
?>