<?php
$str = 'Текст >>123 текст и ещё текст';
$newStr = preg_replace('/>>(\d+)/i', '<a href="#\\1">>>\\1</a>', $str);
print $newStr;
PD9waHAKJHN0ciA9ICfQotC10LrRgdGCID4+MTIzINGC0LXQutGB0YIg0Lgg0LXRidGRINGC0LXQutGB0YInOwoKJG5ld1N0ciA9IHByZWdfcmVwbGFjZSgnLz4+KFxkKykvaScsICc8YSBocmVmPSIjXFwxIj4+PlxcMTwvYT4nLCAkc3RyKTsKCnByaW50ICRuZXdTdHI7