This may sound odd but a right answer might save me hours of coding. I have found a ruby-on-rails class (~10 files, ~1000 lines total) that serves a specific purpose (payment gateway integration). However, I am not familiar with ruby at all and need to use that class in a PHP application. I am wondering if there is a program that can perform the conversion for me. I understand that some portions of code might need to be hand-edited which I can manage.
There are no software out there to do such a thing.
You could interface with it or just take the business logic out of it and redo it from scratch if you're not familiar with Ruby on Rails.
It's quite easy to talk to a Rails app.
You could talk to it through REST calls, which is basically sending a HTTP method to an URL.
For example, a /GET to /products.xml would return you a list of all products. A /PUT to /payment/new with the appropriated params would start a new Payment model, etc.
Take in mind you'd need to know how it works, what models it has and what routes are there available on it. Luckily, Rails comes with many tools. One of them is a rake task called routes.
Execute "rake routes" and you'll be returned a list of all the URLs that Rails works with, with the needed HTTP method and supported params. With this info, you could find out how to talk to that app.
Gotcha: PUT and DELETE are both HTTP methods that currently browsers doesn't implement largely, so Rails uses a _method param. If you send a POST method with a _method:"put" param, it will proceed to act just as a /PUT method was sent.
No, there is no commercial, free, or open source compiler that will take any an arbitrary piece of ruby code and compile it into PHP.
The other answers are suggesting you learn enough ruby-on-rails to create a simple rest framework on top of the existing ruby code, and then use curl (or some other http/web services library) from PHP to fetch and post to URLs in your new simple rails application. These requests would trigger methods in the ruby class, which would run within ruby. There would be no direct eecution of ruby code by the php run time.
My suggestion is you'll spend less time finding payment gateway code written in PHP and using it instead.