What are the performance implications of a php script calling 'exit'?

Question

I've noticed many times where some php scripts exit. It seems to me that this will force an exit of the httpd/apache child (of course another will be started if required for the next request).

But in the CMS, that next request will require the entire init.php initialization, and of course just cleaning up and starting php in the first place.

It seems that the php files usually start with

if ( !defined( 'SMARTY_DIR' ) ) {
include_once( 'init.php' );
}

which suggests that somebody was imagining that one php process would serve multiple requests. But if every script exits, then each php/apache process will serve one request only.

Any thoughts on the performance and security implications of removing many of the exit calls (especially from the most-frequently-called scripts like index.php etc) to allow one process to serve multiple requests?

Thanks, Peter

--ADDENDUM --

Thank you for the answers. That (php will never serve more than one request) is what I thought originally until last week, when I was debugging a config variable that could only have been set in one script (because of the way the path was set up) but was still set in another script (this is on a webserver with about 20 hits/sec). In that case, I did not have a php exit call in the one script that set up its config slightly differently. But when I added the php exit call to that one script (in the alternate directory) this solved the misconfiguration I was experiencing in all my main scripts in the main directory (which were due to having a css directory variable set erroneously, in a previous page execution). So now I'm confused again, because with what all the answers so far say, php should never serve more than one request.

Answer 1

exit does nothing to Apache processes (it certainly doesn't kill a worker!). It simply ends the execution of the PHP script and returns execution to the Apache process, which'll send the results to the browser and continue on to the next request.

The Smarty code you've excerpted doesn't have anything to do with a PHP process serving multiple requests. It just insures that Smarty is initialised at all times - useful if a PHP script might be alternatively included in another script or accessed directly.

Answer 2

Even if you do not call exit your PHP script is still going to end execution, at which point any generated HTML will be returned to the web server to send on to your browser.

The exit keyword allows you to signal to the PHP engine that your work is done and no further processing needs to take place.

Also note that exit is typically used for error handling and flow control - removing it from includes will likely break your application.

Answer 3

I think your confusion comes from what include_once is for. PHP is basically a "shared-nothing" system, where there are no real persistent server objects. include_once doesn't mean once per Apache child, but once per web request.

PHP may hack up a hairball if you include the same file twice. For instance a function with a particular name can only be defined once. This led to people implementing a copy of the old C #ifndef-#define-#include idiom once for each included file. include_once was the fix for this.