function value_from_POST_GET($param_name, $default_value = NULL)
{
    $result = value_from_POST($param_name);
    if (!isset($result)) {
        $result = value_from_GET($param_name, $default_value);
    }
    return $result;
}
<?php

$DIS_Application = "Overview";
$show = value_from_GET('show', 'front');
$DIS_ShowOverview = $show;