SDK incorrectly uses baggage sample-rate for sampling decision when sentry-trace header is missing

[cleptric]

Problem

When the SDK receives a baggage header but no sentry-trace header, it incorrectly uses the sentry-sample_rate value from the baggage to make a sampling decision.

Expected Behavior

When there is no sentry-trace header, the SDK should treat this as a new trace and make an independent sampling decision using the configured traces_sample_rate or traces_sampler. The baggage header without a corresponding sentry-trace header should not influence the sampling decision.

Current Behavior

The SDK extracts the sample rate from the incoming baggage header and uses it to determine whether to sample the transaction, even though the absence of sentry-trace means this is effectively a new trace head.

Why This Matters

• The baggage sample rate reflects the sampling decision of an upstream service that may have different sampling configuration
• Without sentry-trace, there's no parent span to continue, so the local SDK should be the trace head
• This can lead to unexpected sampling behavior where traces are sampled at rates not configured locally

[linear]

PHP-64

[cleptric]

I've created a reproduction for this issue: getsentry/repro#2

You can run it by cloning the repo and following the instructions in the README:

git clone https://github.com/getsentry/repro.git
cd repro/sentry-php/1999
composer install
php reproduce.php