Selected value in same page

When I open the page, $d = "". I select from selectbox the value fex. 45 and I want to gaiv $d the selected value in the same page without refresh it.

if ($_POST['select_day'] == "45") {
    $d = (1 * 86400) + $Stime;
}

And when I click on the button I cannot get $d=45. How can I do it?