In PHP 8.5, the attach(), detach(), and contains() methods of the SplObjectStorage class are deprecated.

[mduchain]

In PHP 8.5, the attach(), detach(), and contains() methods of the SplObjectStorage class are deprecated.
They are replaced by ArrayAccess equivalents to promote a more consistent, array-like syntax for storing and managing objects.

attach($object, $data): Use offsetSet($object, $data) or $storage[$object] = $data instead.

detach($object): Use offsetUnset($object) or unset($storage[$object]) instead.

contains($object): Use offsetExists($object) or isset($storage[$object]) instead.

In the file https://github.com/php-mqtt/client/blob/master/src/Concerns/OffersHooks.php,
the functions register/unregister event handlers are affected

Examples on how to fix:

public function registerLoopEventHandler(\Closure $callback): MqttClient
    {
        $this->loopEventHandlers->**offsetSet**($callback);

        /** @var MqttClient $this */
        return $this;
    }

and

public function unregisterLoopEventHandler(?\Closure $callback = null): MqttClient
    {
        if ($callback === null) {
            $this->loopEventHandlers->removeAll($this->loopEventHandlers);
        } else {
            $this->loopEventHandlers->offsetUnset($callback);
        }

        /** @var MqttClient $this */
        return $this;
    }

The attached file has fixed the issue for me, but I'm only using the LoopEventHandler in my project.

OffersHooks.php

[Namoshek]

This issue has just been resolved by #223.