Storing Page Source As A Variable?

I need to write a script that parses a web page for a given delimiter and then stores the following 50 characters as a variable. I wrote the following php script:

Code:
<?php
$del = "picture_id";
$url = "http://xxxx.net/xxxxx/xxxxxx.php?do=xxxx";
$source = file_get_contents($url);

[Code]....

The delimiter is always going to be constant, however the succeeding characters will be different each and every time. I need to write this in Javascript because it is hosted on my server, and ran by users who post on my brothers message board (which is hosted elsewhere). Since Php is server side I always get "guest" as a value rather than the random string of characters like it should be.