PHP funtion to mask credit card number

cc-mask.php

<?php
/**
 * Masks credit card number leaving up to 4 last available digits.
 *
 * @param string $n     Credit card number (up to 16 characters)
 * @param string $m     Masking string
 * @param int $v        Maximum number of visible characters at the end of the credit card number (in range of 0-4)
 *
 * @return string
 */
function vendor_mask_card_number( $n = '', $m = 'X', $v = 4 ) {
	$n = substr( $n, -16 );
	$v = ( 4 < ( $v = abs( intval( $v ) ) ) ) ? 4 : $v;
	$l = ( ( 16 - $v ) > ( $l = abs( intval( 16 - strlen( $n ) ) ) ) ) ? 12 : $l;
	return substr_replace( str_pad( $n, 16, $m, STR_PAD_LEFT ), str_repeat( $m, $l ), 0, $l );
}

// outputs: XXXXXXXXXXXX3456
$cc_number = '1234567890123456';
echo vendor_mask_card_number( $cc_number );