Description:
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Currently, *by default*, PHP sends *the most restrictive* anti-caching directives possible. 

* no-store is intended as a security measure, not regular cache-control
* must-revalidate alone doesn't prevent caching, just requires browser to revalidate it after it expires. This however, with no-cache prevents use of "offline browsing" feature.
* pre-check and post-check are non-standard directives that let Internet Explorer revalidate cached objects less frequently and/or asynchronously.

Please don't use these directives in the default configuration (called "nocache" cache_limiter), as their use in majority of cases is either unjustified or invalid and causes performance and usability problems (for example: https://bugzilla.mozilla.org/show_bug.cgi?id=261312).

BTW: it turns out that Opera (and most likely other browser vendors) do not fully support these directives *because* PHP abuses them (http://my.opera.com/yngve/blog/2007/02/27/introducing-cache-contexts-or-why-the "This abuse is the reason why must-revalidate is only obeyed for secure sites.")

Reproduce code:
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<?php session_start();


Expected result:
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Cache-control: no-cache


Actual result:
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Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0