<?php
$string = "user is ?user_id=34&first_name=Ralph so is ?user_id=1 also";
$result = preg_replace('/\?(user)_id=(\d+)(.*?)(?! )/i', '/$1/$2/$3', $string );
echo $result ;
PD9waHAKCiRzdHJpbmcgPSAidXNlciBpcyA/dXNlcl9pZD0zNCZmaXJzdF9uYW1lPVJhbHBoIHNvIGlzID91c2VyX2lkPTEgYWxzbyI7CiRyZXN1bHQgPSBwcmVnX3JlcGxhY2UoJy9cPyh1c2VyKV9pZD0oXGQrKSguKj8pKD8hICkvaScsICcvJDEvJDIvJDMnLCAkc3RyaW5nICk7CgplY2hvICRyZXN1bHQgOw==