PHP Warning: session_set_save_handler(): Cannot change save handler when session is active

It looks like you use Yii 1 with PHP 7.2 According to this post and number of other sources like this one and this one, you have a yii bug that can be reported here

The answer from the other topic:

I have done a bug report at php.net and they explained that this is not a bug. Yes in PHP 7.2 a warning is generated now. However this never worked as intended, it just failed silently.

For creating multiple sessions it is required to use session_id(). Have a look at this related question: PHP How can I create multiple sessions?

session_name() as well as session_set_cookie_params() are always nonesense if the session is already running.

For the original answer have a look here: https://bugs.php.net/bug.php?id=75650&thanks=2

Simple fix:

function _read($id)  
   PHP 7.0 -> if is empty return false
   PHP 7.2 -> if is empty return ''

then add @ to other functions and remember to run session_name before session start

@session_name('SOMEID');
@session_start(...
...
@session_set_save_handler(...

It's work for me:

1. Make new class SomeSession in protected/components folder:

class SomeSession extends CCacheHttpSession
{
    public function open()
    {
// don't start new session if session is started now
    if (session_status() === PHP_SESSION_NONE) 
    {
           parent::open();
    }
}

2. in main.php in session section edit:

'session'      => [
            'class'       => SomeSession::class,
            'sessionName' => 'SomeSessionName',
            'autoStart'   => true,
            'cookieMode'  => 'allow',
        ]

3. done =]