session_status()

(PHP 5 >= 5.4.0, PHP 7)

返回当前会话状态

说明

session_status()被用于返回当前会话状态。

返回值

• PHP_SESSION_DISABLED会话是被禁用的。
• PHP_SESSION_NONE会话是启用的，但不存在当前会话。
• PHP_SESSION_ACTIVE会话是启用的，而且存在当前会话。

参见

• session_start() 启动新会话或者重用现有会话

Maybe depending on PHP settings, but if return values are not the above, then go for this:
_DISABLED = 0
_NONE = 1
_ACTIVE = 2

Universal function for checking session status. 

Use always session_status(), to check if a session is already started and active.
if(session_status() !== PHP_SESSION_ACTIVE) session_start();
or 
if(session_status() === PHP_SESSION_NONE) session_start();
Don't use
if(!isset($_SESSION)) session_start();
or
if(session_id() === "") session_start();
They will not work properly after a call to session_write_close().
Both functions will continue to report, that the session exists.
And this is right, you can read from $_SESSION, but if you want to write,
you need session_start() again.
As a shorthand you can use 
@session_start() 
with the @ at the beginning to suppress the 
PHP notice "A session had already been started - ignoring session_start()"
As stated in the manual for session_start(), a second call will do no harm,
it will be simply ignored. But you need the @, if you don't want to get the notice.

The advice of ive_insomnia at live dot com should be taken with great care.
First of all, while his use case for session_status is valid, a simpler way to avoid the warning is:

The example of session_status uses the raw values of constants (2 in this case) created specifically for the purpose of not having to use magic numbers.
Better code would be:

The same can be done using

The use of this function is lies more towards status management: change the behavior of a script when sessions are disabled altogether, for example.

This is how the session_status() works:

鹏仔微信 15129739599 鹏仔QQ344225443 鹏仔前端 pjxi.com 共享博客 sharedbk.com