framework/web/DbSession.php
public function readSession($id)
    {
        $query = new Query();
        $query->from($this->sessionTable)
            ->where('[[expire]]>:expire AND [[id]]=:id', [':expire' => time(), ':id' => $id]);

        if ($this->readCallback !== null) {
            $fields = $query->one($this->db);
            return $fields === false ? '' : $this->extractData($fields);
        }

        $data = $query->select(['data'])->scalar($this->db);
        return $data === false ? '' : $data;
    }
public function readSession($id)
    {
        $query = new Query();
        $query->from($this->sessionTable)
            ->where('[[expire]]>:expire AND [[id]]=:id', [':expire' => time(), ':id' => $id]);

        if ($this->readCallback !== null) {
            $fields = $query->one($this->db);
            return $fields === false ? '' : $this->extractData($fields);
        }

        $data = $query->select(['data'])->scalar($this->db);
        return $data === false ? '' : $data;
    }
Session read handler.
@internal Do not call this method directly.
@param string $id session ID
@return string the session data