Description:
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on php7.4.9 i can unset my session array but on php8.0beta3 i get following error message: "...undefined error key..."

Session: $_SESSION['tmp_image_access']['12345']

Test script:
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if (isset($_SESSION['tmp_image_access']))
            foreach ($_SESSION['tmp_image_access'] as $row) {
                $this->db->query('UPDATE images SET username = ? WHERE token = ?', $this->data['username'], $row);
                session_unset($_SESSION['tmp_image_access'][$row]);
            }

Expected result:
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The session should be deleted with the token '12345'

Actual result:
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i get the following error message on the page "...Undefined array key "12345"..."