How do you replace all characters except last 4 with asterisks in php

Question

I have a variable called $CARD_NUM which can have any amount of numbers more than 4.

How can I replace all but the last 4 characters in this string with asterisks (*)?

For example if:

$CARD_NUM = '123456789012'

The resultant string should be:

********9012

Answer 1

If the variable is guaranteed to be at least 4 digits long, then

$CARD_NUM = "123456789012";
echo str_repeat('*', strlen($CARD_NUM) - 4) . substr($CARD_NUM, -4);

// or, as DougW suggested in a comment, if the string is NOT guaranteed to be at least 4 digits
// echo str_repeat('*', MAX(4, strlen($CARD_NUM)) - 4) . substr($CARD_NUM, -4);

should do it, see it in action!

Answer 2

Try this:

$CARD_NUM = "123456789012";
$replacement = '*';
echo preg_replace('/\d(?=\d{4})/m', $replacement, $CARD_NUM);

If you wanna view 2 or 3 digits only to be displayed just replace 4 in '/\d(?=\d{4})/m' to whatever number of digits you want.