It depends on how you use PHP. If you're using PHP as a module of Apache (mod_php), it's read only when you start, restart or reload Apache. So if you make changes, you need to restart or reload Apache to see them take effect. If you use the CGI though, it's read every time a PHP script is called.
That's for *nix + Apache of course, if you're running it on Windows, I really don't know, although I presume it's the same.