Partial string replacement with fixed width in PHP. Useful for things like credit card numbers.

string-padding.php

<?php
/**
 * Replace middle of the string with given char, keeping length
 *
 * <code>
 * print padString('1234567890123456', 0, 0, '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;
}
?>