Simple PHP script to read an RSS feed

General information for the business: Research
Description of requirements/functionality: We'd like you to write a simple PHP web script for us that fulfils the following contract:

- accepts a URI [rssfeed] and a string [tag] as input parameters
- accesses [rssfeed]
- adds [tag] into the content of each post in [rssfeed] (which may be CDATA encoded)
- outputs the modified [rssfeed] so that readers can effectively subscribe to [rssfeed] through our script, but with the extra [tag] in all the posts

In descriptive terms, the script will act as a proxy for any given rss feed, inserting a specific tag/keyword into each post within the feed.

Background:
We have an existing legacy system which reads RSS feeds and looks for keywords and tags. There are now a number of specific RSS feeds which we'd like to start feeding into our legacy system but we want to 'inject' a keyword so that all posts are picked up by our legacy system.

For example, imagine our legacy system takes in hundreds of pet-related RSS feeds and looks for the keyword Dog. This works fine. But now we want to add another RSS feed which is specifically ABOUT dogs. However, because the feed is so specifically about dogs, people tend not to actually say 'dog' in their posts (it's obvious they're talking about a dog). We know all the posts are about dogs so we want to add the keyword or tag '#dog' to every post in that feed so that our legacy system picks them up and 'counts' them.

Obviously fixing the legacy system to be able to handle different feeds in different ways is the 'correct' solution but unfortunately that is not an option for us.

I hope that explanation of the very simple functionality makes sense.

The script is to be hosted on our own servers so should minimise the use of unusual extensions. The script will be screened for safety prior to being put into production. Any dangerous behaviour of the script (deliberate or unintentional) will result in non-payment of the job. A deliberate trojan horse attempt would have more significant consequences. I know the vast majority wouldn't dream of such a thing but I thought I'd make it nice and clear!

Given the simplicity of the script, we haven't made it more complex by adding memory footprint or performance specifications, however, we would expect the solution to work with any size RSS feed so a stream in / stream out approach would be appropriate, rather than block reading, parsing into memory, manipulating and then block writing.

Thanks!
Specific technologies required: PHP, RSS, XML
Extra notes: