Try to Add a small index.php page in any of the sub folders of your domain and then try to run a small code in it.. If the web browser still downloads the index.php file .. Then check your server apache settings or reset it.Re-enable php..
But you can use the Exec-PHP Plugin .. It will a automatically parse all the php codes inside your post or pages in that way you dont have to write any special codes or tags inside post.
Why you dont want to use a plugin ?
