[Bug] ActivityInterface empty default prefix causes activity type name collision

[koren88]

What are you really trying to do?

Register multiple activity classes on the same worker without activity type name collisions.

Describe the bug

ActivityInterface::$prefix defaults to an empty string, meaning activity type names are derived from method names alone. This makes collisions inevitable when multiple activity classes share method names — and the wrong implementation gets executed silently with no error.

Minimal Reproduction

#[ActivityInterface] // prefix = ''
class ActivityA {
    #[ActivityMethod]
    public function doWork(): string { return 'ActivityA'; }
}

#[ActivityInterface] // prefix = ''
class ActivityB {
    #[ActivityMethod]
    public function doWork(): string { return 'ActivityB'; }
}

Both register as activity type doWork. Calling ActivityA::doWork() may execute ActivityB::doWork() instead.

Environment/Versions

• Temporal PHP SDK 2.14.0
• Docker

Additional context

The workaround is either manually setting a prefix: #[ActivityInterface(prefix: 'classA.')] or explicitly naming every method: #[ActivityMethod(name: 'classA.doWork')].

Both are easy to forget and not prominently documented as required practice.

Suggested fix: Default prefix to the short class name (e.g. ActivityA.) instead of an empty string, similar to the Java SDK behavior.