Mask credit card, account number, etc... with XXXX, show just last 4 last strings/digits.

helper.php

<?php
/**
* You can quick test with http://phptester.net/
**/
$account_number = '123123123';
$credit_card_number = '4242424242424242';

$account_length = strlen($account_number);
$credit_card_length = strlen($credit_card_number);

$account_output = substr_replace($account_number, str_repeat('X', $account_length - 4), 0, $account_length - 4);

echo '<pre>';
echo $account_output; // XXXXX3123
echo '</pre>';

$credit_card_output = substr_replace($credit_card_number, str_repeat('X', $credit_card_length - 4), 0, $credit_card_length - 4);

echo '<pre>';
echo $credit_card_output; // XXXXXXXXXXXX4242
echo '</pre>';