Session unset unsetting every variable?

I only want to unset play_mode however the second line also unsets landing_site.

Why is this happening?

session_start();

session_unset($_SESSION['play_mode']);

echo $_SESSION['la开发者_JAVA技巧nding_site'];

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Because, as per the documentation,

The session_unset() function frees all session variables currently registered.

Also, session_unset() takes no arguments.

If you only want to unset $_SESSION['play_mode'], then just do unset($_SESSION['play_mode']).

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session_unset
The session_unset() function frees all session variables currently registered.
Try this instead:

unset($_SESSION['play_mode']);

[edit]
This question needs more people linking the manual!

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From php.net

void session_unset ( void )
The session_unset() function frees all session variables currently registered.

If you want to unset just one variable then just use unset.

unset($_SESSION['play_mode']);

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you want unset() for a single var not session_unset() the whole session