How can i get my exact age? i make a code but it doesn't work. i want to happen is calculate the age. date now - date of birth then will show my age.

Code:
<?php
$day = $_POST['day'];          //using select or drop down list
$month = $_POST['month'];   //using select or drop down list
$year = $_POST['year'];       //using select or drop down list
$age = date(("m-d-Y") - ($month.$day.$year));

echo "<h1>Your age now is $age </h1>"; 
?>