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mixed preg_replace_callback ( mixed $pattern , callable $callback , mixed $subject [, int $limit = -1 [, int &$count ]] )

<?php
// Increment the year in the text by one.
$text = "April fools day is 04/01/2002\n";
$text.= "Last christmas was 12/24/2001\n";
// Callback function
function next_year($matches)
{
  // Usually: $matches[0] is the complete match
  // $matches[1] is the match for the first captured group
  // and so on
  return $matches[1].($matches[2]+1);
}
echo preg_replace_callback(
            "|(\d{2}/\d{2}/)(\d{4})|",
            "next_year",
            $text);
?>

April fools day is 04/01/2003
Last christmas was 12/24/2002