how to convert date("u") to string date in php code example

Example: change datetime format from Y-m-d h:i:s to d-m-Y in php

$old_date = date('l, F d y h:i:s');              // returns Saturday, January 30 10 02:06:34
$old_date_timestamp = strtotime($old_date);
$new_date = date('Y-m-d H:i:s', $old_date_timestamp);