CreditCardEncryption.php

<?php

function encrypt($str, $num){
    $len = strlen(substr($str, $num, strlen($str) - $num));
     $goin = str_repeat('*', $len);
     $res =  substr($str, 0 , $num) .$goin . substr($str, $len, strlen($str)); 
    return $res;
 }

 print(encrypt("1111111111111111111111", 4)); //1111******************1111

 ?>