Fix isOpen() compatibility with PHP 8.0+ Socket object

[rajesh-varatharajan]

Problem

In PHP 8.0+, socket_create() returns a \Socket object instead of a
resource. The isOpen() check is_resource($this->socket) always
returns false on PHP 8.0+, causing the connection to appear closed
even when it is open.

Fix

Replace:

if (!is_resource($this->socket)) {
With:
if (!is_resource($this->socket) && !is_object($this->socket)) {

Using is_object() instead of instanceof \Socket ensures backward
compatibility with PHP 5.4+ (the minimum required version of this
package), since the \Socket class does not exist before PHP 8.0.

- PHP < 8.0: socket is a resource → is_resource() returns true
- PHP 8.0+: socket is a \Socket object → is_object() returns true