preg_replace_callback

<?php
  // this text was used in 2002
  // we want to get this up to date for 2003
  $text = "April fools day is 04/01/2002\n";
  $text.= "Last christmas was 12/24/2001\n";

  // the callback function
  function next_year($matches) {
    // as usual: $matches[0] is the complete match
    // $matches[1] the match for the first subpattern
    // enclosed in '(...)' and so on
    return $matches[1].($matches[2]+1);
  }

  echo preg_replace_callback(
              "|(\d{2}/\d{2}/)(\d{4})|",
              "next_year",
              $text);

  // result is:
  // April fools day is 04/01/2003
  // Last christmas was 12/24/2002
?>

<?php
  /* a unix-style command line filter to convert uppercase
   * letters at the beginning of paragraphs to lowercase */

  $fp = fopen("php://stdin", "r") or die("can't read stdin");
  while (!feof($fp)) {
      $line = fgets($fp);
      $line = preg_replace_callback(
          '|<p>\s*\w|',
          create_function(
              // single quotes are essential here,
              // or alternative escape all $ as \$
              '$matches',
              'return strtolower($matches[0]);'
          ),
          $line
      );
      echo $line;
  }
  fclose($fp);
?>