feat: add before send callback

[marandaneto]

💡 Motivation and Context

Customers need a configuration hook to inspect, modify, or drop events before upload, matching the before-send behavior available in other PostHog SDKs.

This adds a before_send client option. It receives the fully enriched event array after SDK metadata, server attribution, groups, timestamps, and feature flag properties have been applied. Returning null drops the event.

💚 How did you test it?

• ./vendor/bin/phpunit --no-coverage --filter 'BeforeSend|BatchSizeOne' test/PostHogTest.php
• composer api:check

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with the pi coding agent. The callback runs immediately before queueing the fully enriched capture payload; callbacks that return null, return a non-array, or throw will drop only that event.

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat: add before send callback" | Re-trigger Greptile}

[github-actions]

posthog-php Compliance Report

Date: 2026-07-06 07:13:52 UTC
Duration: 95356ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	13ms
Format Validation.Event Has Uuid	✅	6ms
Format Validation.Event Has Lib Properties	✅	6ms
Format Validation.Distinct Id Is String	✅	6ms
Format Validation.Token Is Present	✅	5ms
Format Validation.Custom Properties Preserved	✅	6ms
Format Validation.Event Has Timestamp	✅	5ms
Retry Behavior.Retries On 503	✅	5317ms
Retry Behavior.Does Not Retry On 400	✅	2010ms
Retry Behavior.Does Not Retry On 401	✅	2010ms
Retry Behavior.Respects Retry After Header	✅	8019ms
Retry Behavior.Implements Backoff	✅	15723ms
Retry Behavior.Retries On 500	✅	5116ms
Retry Behavior.Retries On 502	✅	5117ms
Retry Behavior.Retries On 504	✅	5117ms
Retry Behavior.Max Retries Respected	✅	16533ms
Deduplication.Generates Unique Uuids	✅	13ms
Deduplication.Preserves Uuid On Retry	✅	5114ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10323ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5118ms
Deduplication.No Duplicate Events In Batch	✅	13ms
Deduplication.Different Events Have Different Uuids	✅	7ms
Compression.Sends Gzip When Enabled	✅	6ms
Batch Format.Uses Proper Batch Structure	✅	7ms
Batch Format.Flush With No Events Sends Nothing	✅	3ms
Batch Format.Multiple Events Batched Together	✅	9ms
Error Handling.Does Not Retry On 403	✅	2008ms
Error Handling.Does Not Retry On 413	✅	2010ms
Error Handling.Retries On 408	✅	5117ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	11ms
Request Payload.Flags Request Uses V2 Query Param	✅	6ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	6ms
Request Payload.Flags Request Omits Authorization Header	✅	6ms
Request Payload.Token In Flags Body Matches Init	✅	4ms
Request Payload.Groups Round Trip	✅	6ms
Request Payload.Groups Default To Empty Object	✅	5ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	5ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	6ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	6ms
Request Lifecycle.No Flags Request On Init Alone	✅	2ms
Request Lifecycle.No Flags Request On Normal Capture	✅	6ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	8ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	5ms
Retry Behavior.Retries Flags On 502	✅	108ms
Retry Behavior.Retries Flags On 504	✅	109ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	33ms

[greptile-apps]

_{Reviews (2): Last reviewed commit: "test: cover before send edge cases" | Re-trigger Greptile}

[marandaneto]

Closes #178

[ioannisj]

Approving to unblock but currently before_send just runs for capture() and skips identify/alias or any event that doesn't have capture as an entrypoint