preg_replace_callback

具体的な使い方はphpマニュアル

を参照
例1

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

    //   ： 
    $text = "April fools day is 04/01/2002
Last christmas was 12/24/2001
";

    //     
    function nextYear($matches)
    {
        //  dump($matches);
        /*
         * array:3 [
         *  0 => "04/01/2002"
         *  1 => "04/01/"
         *  2 => "2002"
         * ]
         *
         * array:3 [
         *  0 => "12/24/2001"
         *  1 => "12/24/"
         *  2 => "2001"
         * ]
         *
         *                   
         */

        //   ：
        // $matches[0]       
        // $matches[1]            
        return $matches[1].($matches[2]+1);
    }

    $count = 0;
    // April fools day is 04/01/2003
Last christmas was 12/24/2002

    echo preg_replace_callback("|(\d{2}/\d{2}/)(\d{4})|","nextYear",$text, -1, $count);
    // 2
    var_dump($count);