Why is the standard session lifetime 24 minutes (1440 seconds)?

Question

I've been doing some research on PHP Session Handling and came across the session.gc_maxlifetime value of 1440 seconds. I've been wondering why the standard value is 1440 and how it is calculated? What is the basis for this calculation?

How long does it make sense to keep sessions? What min/max values for session.gc_maxlifetime would you recommend? The higher the value, the more vulnerable the Web-App is for Session Hijacking, I'd say.

Solution

The real answer is probably very close to this:

Back during PHP3 days, PHP itself had no session support.

But an open-source library called PHPLIB, initially written by Boris Erdmann and Kristian Koehntopp from NetUSE AG, provided sessions via PHP3 code.

Session lifetimes were defined in minutes, not seconds. And the default lifetime was 1440 minutes, or exactly one day. Here's that line of code from PHPLIB:

var $gc_time  = 1440;       ## Purge all session data older than 1440 minutes.

Sascha Schumann was involved with the PHPLIB project around the period of 1998 to 2000. There's no doubt he was familiar with the PHP3 session code.

Then PHP4 came out in the year 2000 with native session support, but now the lifetime was specified in seconds.

I'll bet someone just never bothered converting minutes to seconds. It's probable that person was Sascha Schumann. Once that value was coded into the Zend engine, it became the configuration (php.ini) default as well.

Comments

[Praveen Kumar Purushothaman]

+1 Nice question. Something related to, 60 x 12 x 2... Lets wait for interesting answers... :)

[Álvaro González]

Here's the source code line where default value is set. Those familiar with GIT may be able to track its history and maybe find a RFC or bug ticket (if any).

[Anil Namde]

stackoverflow.com/questions/156712/… might help some earlier discussion done on this

[Anna Völkl]

@Anil: This discussion does not answer my question.

[Dai]

@ÁlvaroG.Vicario I found the change: github.com/php/php-src/commit/… however there is zero relevant associated documentation. 1440 was indeed the original number of minutes for the timeout, so I guess we'll never know unless someone tracks down Sascha Schumann.

[Dai]

I found Sascha's email address and contacted him about this, I'll let people know if he responds.

[Dennis]

Why didn't Sascha respond!?

[Félix Adriyel Gagnon-Grenier]

so why is it the standard session lifetime?

[dimitar veselinov]

Because its a math nerd number thats approximately the same as what research shows is the largest time between two request in the same session (20-30 minutes).

[Markus Malkusch]

It strikes to one's eyes that 24 gets a special meaning when changing the unit. So out of the given answers yours makes the most sense to me.

[Markus Malkusch]

@dimitarveselinov Which research?

[SysVoid]

That's interesting. Thanks for sharing! :)

[Anna Völkl]

This is really interesting! Thanks!

[CXJ]

One reference to PHP4 default settings: github.com/php/php-src/blob/PHP-4.0/ext/session/session.c

[dr_hanns]

According to PHP documentation the session lifetime is set in seconds: php.net/manual/en/…