feat: add caller-supplied default value to FeatureFlagEvaluations::isEnabled()

[posthog]

💡 Motivation and Context

Brings posthog-php into compliance with the is-feature-enabled spec in PostHog/sdk-specs, which is a hard SHALL with no server-SDK carve-out: "The SDK SHALL accept a caller-supplied boolean default ... and SHALL return it whenever the flag has no value ... A flag that has a value — including false — always wins over the caller-supplied default."

Per the compliance audit (compliance/posthog-php.md#n8), the legacy Client::isFeatureEnabled() lacks this, but that method is already deprecated in favor of Client::evaluateFlags() + FeatureFlagEvaluations::isEnabled(). The canonical, non-deprecated FeatureFlagEvaluations::isEnabled(string $key): bool has the same gap: it hardcodes ?? false on a miss, so a genuinely-disabled flag and an unresolved/unknown flag are indistinguishable, and there's no way for a caller to opt into a true fallback.

This PR fixes the canonical path (not the deprecated legacy method, per guidance not to invest further in deprecated APIs) by adding an optional defaultValue parameter.

What changed

• FeatureFlagEvaluations::isEnabled(string $key, bool $defaultValue = false): bool — the default is returned only when the flag has no resolvable value; any real value (including false) still wins.
• Added .changeset/feature-flag-evaluations-default-value.md per this repo's release process.
• Updated api/public-api.json via composer api:update to reflect the new optional parameter.

Backwards compatibility

Purely additive — the new parameter is optional and defaults to false, which is exactly today's unconditional fallback behavior. No existing caller's behavior changes.

💚 How did you test it?

• Added two new test cases to test/FeatureFlagEvaluationsTest.php:
  • testIsEnabledReturnsCallerSuppliedDefaultForUnknownKey — unknown key returns the caller-supplied default (true/false), and still defaults to false when omitted.
  • testIsEnabledDefaultDoesNotOverrideARealFalseValue — a flag that resolves to a real false is not overridden by defaultValue: true.
• Ran the full suite: vendor/bin/phpunit — 447 tests, 3808 assertions, all passing.
• Ran composer api:check (via scripts/check-public-api.php) to confirm the only public API delta is the new optional parameter, then composer api:update to refresh the snapshot.
• Ran phpcs --standard=phpcs.xml against the changed files — clean.

📝 Checklist

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

If releasing new changes

• Added a .changeset/*.md change intent file

🤖 Agent context

Autonomy: Fully autonomous

This PR was produced by an unattended, scheduled agent run ("Daily SDK Compliance Implementer") that reads the compliance matrices in PostHog/sdk-specs and implements one backward-compatible remediation per run. The candidate was selected from the matrix's global open-gaps list, cross-checked against this repo's open PRs/branches for duplicates (none found — an old, already-merged 2022 branch, ff-remove-defaultvalue, was a false-positive name match, unrelated to this change), and implemented directly against the canonical (non-deprecated) isEnabled() API rather than the deprecated legacy method.

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Created with PostHog Code

[github-actions]

posthog-php Compliance Report

Date: 2026-08-08 16:08:33 UTC
Duration: 95255ms

✅ All Tests Passed!

46/46 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	11ms
Format Validation.Event Has Uuid	✅	5ms
Format Validation.Event Has Lib Properties	✅	5ms
Format Validation.Distinct Id Is String	✅	5ms
Format Validation.Token Is Present	✅	4ms
Format Validation.Custom Properties Preserved	✅	5ms
Format Validation.Event Has Timestamp	✅	5ms
Retry Behavior.Retries On 503	✅	5315ms
Retry Behavior.Does Not Retry On 400	✅	2009ms
Retry Behavior.Does Not Retry On 401	✅	2009ms
Retry Behavior.Respects Retry After Header	✅	8015ms
Retry Behavior.Implements Backoff	✅	15729ms
Retry Behavior.Retries On 500	✅	5116ms
Retry Behavior.Retries On 502	✅	5112ms
Retry Behavior.Retries On 504	✅	5116ms
Retry Behavior.Max Retries Respected	✅	16529ms
Deduplication.Generates Unique Uuids	✅	11ms
Deduplication.Preserves Uuid On Retry	✅	5113ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10322ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5113ms
Deduplication.No Duplicate Events In Batch	✅	11ms
Deduplication.Different Events Have Different Uuids	✅	5ms
Compression.Sends Gzip When Enabled	✅	5ms
Batch Format.Uses Proper Batch Structure	✅	5ms
Batch Format.Flush With No Events Sends Nothing	✅	3ms
Batch Format.Multiple Events Batched Together	✅	9ms
Error Handling.Does Not Retry On 403	✅	2007ms
Error Handling.Does Not Retry On 413	✅	2009ms
Error Handling.Retries On 408	✅	5115ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	6ms
Request Payload.Flags Request Uses V2 Query Param	✅	5ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	4ms
Request Payload.Flags Request Omits Authorization Header	✅	4ms
Request Payload.Token In Flags Body Matches Init	✅	4ms
Request Payload.Groups Round Trip	✅	4ms
Request Payload.Groups Default To Empty Object	✅	4ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	4ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	4ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	4ms
Request Lifecycle.No Flags Request On Init Alone	✅	2ms
Request Lifecycle.No Flags Request On Normal Capture	✅	5ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	6ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	4ms
Retry Behavior.Retries Flags On 502	✅	107ms
Retry Behavior.Retries Flags On 504	✅	107ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	8ms