Scope::getTransaction returns null when Transaction isn't sampled

[blackwolf12333]

Environment

How do you use Sentry?
Sentry SaaS (sentry.io)

Which SDK and version?
sentry-php@3.4.0

Steps to Reproduce

1. initialize sentry with a very low sample rate for easier repro
2. call $transaction = startTransaction
3. call SentrySdk::getCurrentHub()->setSpan($transaction)
4. call SentrySdk::getCurrentHub()->getTransaction()

Expected Result

I expected to get the instance of the Transaction I just started.

Actual Result

getTransaction returned null because when you call startTransaction and this transaction isn't getting sampled it does not initialize the SpanRecorder (Hub.php:256). However, getTransaction gets the transaction from Scope which checks for an active SpanRecorder and returns null otherwise.

This behavior is not documented anywhere, I think it would suffice to add a note to the Hub::getTransaction method that if it's not sampled it will return null.

[ste93cry]

I checked the code of the JS SDK (the code of this package is mainly a porting from it) and it looks like the same problem you are describing was also present there and it was considered a bug which got fixed in getsentry/sentry-javascript@5ac0fca. I will try to open a PR soon to fix it here too