Apparently, you just need to run:
exec (screen ./tserver);
http://php.net/manual/en/function.exec.php
I haven't used PHP for many years, so I had to look it up.
May I give you some advice? When I used PHP, a long time ago, I was a computer/linux/programming newbie and didn't know any better. PHP was the only language that I knew, so I ventured into "shell scripting" with PHP. It was very good practice, but I eventually realized that it's hardly a good idea. There are many languages a lot more suitable to such tasks.
Happy coding!