Good news for you, it's all real easy now. Download and install RedHat 6.2, use the server install if it's only going to be a server. You can probably stop there, or better yet, use the Apache Tool Box to install Apache, PHP, and mucho other useful Apache modules (http://www.apachetoolbox.com/).
Informix might be trickier, but start here, this should help (http://www.informix.com/evaluate/).
P.S. did you know that Informix developers are the highest paid on average? I figured Oracle or DB2 would be the top. Heard it from a friend. He's a MS SQL guy. Too bad for him.
Good luck. I've done this numerous times. Feel free to contact me for more info.