Hi maigre,
I may be misunderstanding what you are looking for, but take a look at the html iframe element.
There are a lot of problems with this solution, and it may not work for your particular application. I try to avoid iframes whenever possible, but there are times that they serve a valid and necessary purpose.
You can style an iframe to completely blend into your page, nearly undetectable (visually). HOWEVER, it is a completely foreign entity to the page and anything inside the iframe (forms, etc) are separate and unusable by the page it is embedded in. Even the parent pages css will not apply to the elements in the iframe. It functions the same as a separate browser window that is imbedded into your existing page.
Now, here is the other thing that you and the host should be aware of: PHP 4.3 (I know you said you weren't sure of the version) was released on December 27, 2002. It is 10 years old and [b]should not be used[/b] for any commercial application as far as I am concerned. There is no excuse that I can think of for a host to still be running 4.3 or frankly 4.anything (5 was released in 2004) and I would really push to have them update it.
If you have no other option, I would look at iframes or rewriting the library (if possible). Either way, there have been a substantial number of security enhancements and bug fixes in php since the version you will be using and I would expect the site to be much more vulnerable than it otherwise would!
The last thing I would say is in the event you are forced to stay with an old php version, let me know and I’ll do what I can to help you integrate whatever solution you decide to pursue.
jay