In PHP, is there any harm in running session_start() multiple times?

Question

Presumably there\'s some tiny performance hit, but beyond that?

Answer 1

From the docs:

As of PHP 4.3.3, calling session_start() after the session was previously started will result in an error of level E_NOTICE. Also, the second session start will simply be ignored.

So no, it won't "cause harm", but it'll throw an error. And the fact that it's happening is probably an indicator that you're doing something incorrectly, and might need to re-think how your code is laid out.