src/Connection.php
public function close()
    {
        if ($this->_socket !== false) {
            $connection = ($this->unixSocket ?: $this->hostname . ':' . $this->port) . ', database=' . $this->database;
            \Yii::trace('Closing DB connection: ' . $connection, __METHOD__);
            try {
                $this->executeCommand('QUIT');
            } catch (SocketException $e) {
                // ignore errors when quitting a closed connection
            }
            fclose($this->_socket);
            $this->_socket = false;
        }
    }
public function close()
    {
        if ($this->_socket !== false) {
            $connection = ($this->unixSocket ?: $this->hostname . ':' . $this->port) . ', database=' . $this->database;
            \Yii::trace('Closing DB connection: ' . $connection, __METHOD__);
            try {
                $this->executeCommand('QUIT');
            } catch (SocketException $e) {
                // ignore errors when quitting a closed connection
            }
            fclose($this->_socket);
            $this->_socket = false;
        }
    }
Closes the currently active DB connection.
It does nothing if the connection is already closed.