I've set up a few Apache servers on Debian at home before so I might be able to help with this. If the device is already set up with Debian then it hopefully already has the necessary packages on it but possible not. Hopefully it at least has SSH so if you have it hooked to a router, you can log onto the router and get the local IP of the device. Then you can open a terminal in another computer connected on the same router (does the iPhone do SSH?) and just log onto it by running "SSH xxx.xxx.xxx.xxx". The local IP will probably be something like 10.0.0.2.
Once you've done that, you can run any updates you want from there and install Apache. The commands to install something in Debian is "sudo apt-get install XXXXX" for whichever package you want so you can run "sudo apt-get install apache2" and then "sudo apt-get install php5" or you can install them all at once like "sudo apt-get install apache2 php5 mysql" and let it go. The problem here is you'll need the root (sudo) password for the device which is only specified when Linux is first installed. But if you can log onto the device and access its files, I'll tell you how to hack it to change the password.
If there isn't already a version of Linux on it, you might want to look at Damn Small Linux or Puppy Linux, they're both built to run on less than 100 megs of RAM and take up very little disk space.