Partial string replacement to anonymize credit card numbers, bank account numbers etc

Partial-string-replacement.php

/**
 * Replace middle of the string with given char, keeping length
 * http://mamchenkov.net/wordpress/2011/05/17/partial-string-replacement-with-fixed-length-in-php/
 *
 * <code>
 * print padString('1234567890123456', 1, 4, 'x');
 * </code>
 *
 * @param string $string String to process
 * @param integer $start Characters to skip from start
 * @param integer $end Characters to leave at the end
 * @param string $char Character to replace with
 * @return string
 */
function padString($string, $start, $end, $char) {
    $result = '';
 
    $length = strlen($string) - $start - $end;
    if ($length <= 0) {
        $result = $string;
    }
    else {
        $replacement = sprintf("%'{$char}" . $length . "s", $char);
        $result = substr_replace($string, $replacement, $start, $length);
    }
 
    return $result;
}