how to create a persistent USB serial connection between raspberry pi & arduino?

Projects Interfacing w/ Software on the Computer
1

I have a raspberry pi connected via USB to an arduino. I can build and upload from the pi using inotool.
I want a mini server to connect to the arduino, maintain the connection, and pass strings back and forth.
I want to be able to send instructions to the arduino from php. I'm guessing I need to pipe using exec()?
I want to be able to get messages back to PHP from the mini server. At the speed I'm going the time delay is not a big deal.

I've found many tutorials about listening to serial in linux. I haven't found any yet about sending.

Can you help me, please?

2

I've found many tutorials about listening to serial in linux. I haven't found any yet about sending.

The serial port is a file, as far as linux is concerned. You can read from it. You can write to it.