Just to confirm what Fred is saying, there's really no way this can be done and no reasonable way Eloqua (the company) could make it possible even if they wanted to. I don't think Eloqua (the application) is written in PHP and it's probably not running in a server environment in which it's even possible to run PHP. To do this, they would have to make it possible to run PHP from within the Eloqua application inside some sort of virtualized environment that would be very difficult to develop, and it would be really hard to do it without creating a huge security risk. Even if all of that could be done, it would be a huge cost and effort for minimal benefit.
If you need to run server-side scripts, probably the best way to do it would be to host the script on your own server, and use JavaScript on the Eloqua landing page to call the script and output the results.
In the vast majority of cases it's probably way easier to either do what you want to do using JavaScript only, or host the entire landing page on your own server where you can do whatever you want.