We can utilise the second argument of strtotime to specify the date to calculate “last Friday” and other relative dates from:
<?php
echo date("Y-m-d H:i:s", strtotime("last Friday", strtotime("2016-01-11"));
?>
Similarly,
<?php
echo date("Y-m-d H:i:s", strtotime("-10 days", strtotime("2016-01-11");
?>
Cheers!