I am writing a Wordpress plug in php in and next step is some kind of add on to this plug in.
The add on would scrape data from web, sending forms etc. I have this
A WordPress plugin that depends on Ruby isn't going to be portable. That's OK if you're the only one who will be using it, though.
If the Ruby script needs to return a result that will be used immediately by the PHP script that's calling it, then something like exec() is the only way. Make sure you escape any arguments you pass to the Ruby script; otherwise you'll be vulnerable to injection attacks.
If the Ruby script doesn't need to return a result immediately (e.g. some background processing, such as thumbnail generation) then I think the best way would be for the PHP script to insert a row into a MySQL database or something similar. The Ruby script can work in the background or run from cron, check the database periodically for new jobs, and do whatever processing it needs to do. This approach avoids the performance overhead and security issues of exec(), and it's arguably also more scalable. (A similar approach would have the Ruby script listen on a socket, and your PHP scripts would connect to the socket. But this requires more work to get it right.)