'Getting some error like "Warning: Undefined array key" in my website in php [duplicate]

I'm doing validate session I'm getting some field is "Warning: Undefined array key", This issue is happened now warning error my php code is blow i have given please what is i'm doing wrong...thanks...

These fields are I'm getting the error:

Warning: Undefined array key "lastActiveTime"
Warning: Undefined array key "USERNAME" 
Warning: Undefined array key "SESSION_ID" 
Warning: Undefined array key "HTTP_USER_AGENT"
Warning: Undefined array key "REMOTE_ADDR"

php code :

function validate_session()
{
    //error_reporting(E_ERROR | E_PARSE);
        $time_outvalue = 18000;  // for session timeout

        $now = time();
        $diff = $now - $_SESSION['lastActiveTime'];
        $username = $_SESSION['USERNAME'];
        $session_id = $_SESSION['SESSION_ID'];
        if(($diff >= $time_outvalue && isset($_SESSION['lastActiveTime'])) || (($_SESSION['HTTP_USER_AGENT'] != md5($_SERVER['HTTP_USER_AGENT'])) && ($_SESSION['REMOTE_ADDR'] != $_SERVER['REMOTE_ADDR']))){            // Session timeout

            setcookie("PHPSESSID", "", time()-3600);
            unset($_SESSION['lastActiveTime']);
            unset($_SESSION['username']);
            session_destroy();

        }

        $_SESSION['lastActiveTime'] = time();
}
php


Solution 1:[1]

$_SESSION creates after session_start() (or if session.auto_start enabled in php.ini), you can add something like this isset($_SESSION) || session_start(); at begin.

Sources

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Source: Stack Overflow

Solution Source
Solution 1 WinterSilence

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