<?php

$re = '/\{\$([^.{}]+(?:\.[^.{}]+)+)\}/'; 
$str = "Welcome {\$user.first_name} to the {\$site} version 1.5. Your username is {\$user.username}. Your reputation at present is {\$user.reputation.name}"; 
preg_match_all($re, $str, $matches);
print_r($matches[1]);