You posted all of your code. I pointed out why that wouldn't work, so you posted a snippet of your revised code. Posting snippets is not the way to get help. Post all of your code.
It is likely that when the PHP parser ends, that it closes the serial port. So, when you execute the script again (how you are doing is a bit of a mystery, since there is no form with submit buttons), with a different action, the port you are trying to write to is not open.
Do you have warnings/errors in PHP enabled?