<?php
$worked = (time() - strtotime('24-06-2015'))/YEAR_IN_SECONDS; // date is start date
echo "Adam has been with for us for $worked years";
?>

The number will be something like 3.9811951737697 years, less than ideal. This concept clearly needs further refinement. It would be a matter of using a different divisor and doing some additional arithmetic and rounding. Maybe start by using DAY_IN_SECONDS instead.