I thinks it has something to do with that command not actually opening up the serial port. Which seems strange, I have had issues on windows machines sharing the same serial connection (the same setup on a mac didn't have any problems), in this case you could try opening up the serial port in the bash and see if that does it then you can point to the php code not working correctly.
On a side note I am attempting to do something similar although a bit more complicated, running node.js on a local host to create a server which will return the current values of the serial information if asked for.
I am doing it using:
I have also had success with pyserial
I have an arduino project up on google code using this here.