session bug?

I used session in many places.
when I set(update) part of session. the previous session data was destroied. only the updated data are available.

please test it!!
step one, use the follow data.

PHP Code:

$data = array(
        'userid'  => '1',
        'username'  => 'johndoe',
        'email'     => '[email protected]'
);

$session->set$data); 


step two, run the follow code to update part of data.

PHP Code:

$newdata = array(
        'username'  => 'david name',
);

$session->set($newdata); 


now the session data is array('username'=>'david name')
the session of email and userid are not available.

is it a bug?

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