php preg replace not in pattern examples

Example 1: preg_replace

<?php
$string = 'April 15, 2003';
$pattern = '/(\w+) (\d+), (\d+)/i';
$replacement = '${1}1,$3';
echo preg_replace($pattern, $replacement, $string);

Example 2: php preg replace

preg_replace($pattern, $replacement, $subject [, $limit [, &$count]]);
// Returns an array if the subject parameter is an array,
// or a string otherwise.
// If matches are found, the new subject will be returned, otherwise
// subject will be returned unchanged or NULL if an error occurred.