- substr: documentation ( source)
- str_repeat: documentation ( source)
<?php
//echo date('m/d/Y + 3 days');
//echo date('F d Y', strtotime(date('m/d/Y') . ' + 4 days'));
$accNo = '123456789';
$masked = (substr($accNo, -3));
echo str_repeat('X', 12 - strlen($masked)).$masked;