Adding X's to variable

[robsgaming]

I need PHP to format a credit card number for me.

This is for a credit card number that I'm going to blanket out all the numbers but the first four digits.

For example the credit card number might be:

1234-1234-1234-1234

I want PHP to automatically format the number after it's entered to look like this:

XXXX-XXXX-XXXX-1234

Thanks in advance for any help. I'm stuck on trying to figure this out.

[The_Chancer]

function returnSafeCard($card)
{
     $newcard=substr($card,-4);
     $add="XXXX-XXXX-XXXX-".$newcard;
     return $add;
}

Something like that ?

Or maybe a regexp could help (no idea on those though.....)