fix(flags): align local string matching with the flags service

[marandaneto]

💡 Motivation and Context

Local feature flag evaluation did not match the /flags service for several case-insensitive string comparisons. Exact matching used ASCII-only lowercasing, integral floats such as 323.0 lost their decimal suffix, and not_icontains treated a match at offset zero as no match.

This fixes PostHog/posthog#78019 by:

• using simple Unicode lowercase conversion for exact and is_not, including array membership
• keeping icontains, prefix, and suffix operators on the existing ASCII-only lowercase path
• preserving backend-compatible finite float formatting, including integral and scientific values
• preserving the previous behavior for non-finite PHP floats instead of throwing
• using strict strpos comparison for not_icontains
• adding symfony/polyfill-mbstring and symfony/polyfill-iconv so Unicode lowercase conversion works without either native extension, including context-independent sigma and U+0130 expansion semantics

A patch changeset is included for posthog-php.

💚 How did you test it?

• vendor/bin/phpunit --no-coverage test/FeatureFlagLocalEvaluationTest.php - 120 tests and 2,578 assertions passed
• vendor/bin/phpunit --no-coverage - 479 tests and 3,904 assertions passed
• vendor/bin/phpcs --warning-severity=0 - no errors
• composer validate --no-check-publish - valid, with the existing package version warning
• composer api:check - public API snapshot is up to date
• Composer resolution and Unicode matching were validated with native ext-mbstring and ext-iconv disabled

PHPUnit reported existing warnings and deprecations, but no test failures.

📝 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 change to generate a change intent file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

Implemented with Pi using the PR and autoreview skills. The reviews identified scientific float formatting, contextual Greek sigma behavior, and the missing MB_CASE_LOWER_SIMPLE constant without native mbstring. These were fixed with regression coverage. Exact matching uses native simple lowercase when available and the polyfill’s context-independent lowercase mapping otherwise, with explicit U+0130 expansion to match the Rust backend.

[github-actions]

posthog-php Compliance Report

Date: 2026-08-27 06:46:44 UTC
Duration: 95318ms

✅ 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	✅	5ms
Format Validation.Distinct Id Is String	✅	6ms
Format Validation.Token Is Present	✅	6ms
Format Validation.Custom Properties Preserved	✅	6ms
Format Validation.Event Has Timestamp	✅	5ms
Retry Behavior.Retries On 503	✅	5316ms
Retry Behavior.Does Not Retry On 400	✅	2009ms
Retry Behavior.Does Not Retry On 401	✅	2010ms
Retry Behavior.Respects Retry After Header	✅	8014ms
Retry Behavior.Implements Backoff	✅	15730ms
Retry Behavior.Retries On 500	✅	5111ms
Retry Behavior.Retries On 502	✅	5115ms
Retry Behavior.Retries On 504	✅	5114ms
Retry Behavior.Max Retries Respected	✅	16529ms
Deduplication.Generates Unique Uuids	✅	11ms
Deduplication.Preserves Uuid On Retry	✅	5113ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10315ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5117ms
Deduplication.No Duplicate Events In Batch	✅	12ms
Deduplication.Different Events Have Different Uuids	✅	7ms
Compression.Sends Gzip When Enabled	✅	6ms
Batch Format.Uses Proper Batch Structure	✅	6ms
Batch Format.Flush With No Events Sends Nothing	✅	3ms
Batch Format.Multiple Events Batched Together	✅	10ms
Error Handling.Does Not Retry On 403	✅	2008ms
Error Handling.Does Not Retry On 413	✅	2010ms
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	✅	7ms
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	✅	5ms
Request Payload.Token In Flags Body Matches Init	✅	5ms
Request Payload.Groups Round Trip	✅	4ms
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	✅	4ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	5ms
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	✅	7ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	5ms
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

[greptile-apps]

Prompt To Fix All With AI

### Issue 1
composer.json:20
**Iconv becomes mandatory**

If a PHP 8.2+ installation lacks `ext-iconv`, requiring `symfony/polyfill-mbstring` makes Composer reject the installation even when native `ext-mbstring` is available, so environments that previously satisfied the SDK's declared requirements can no longer install or upgrade it.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "fix(flags): use simple Unicode lowercase..." | Re-trigger Greptile}

[arnohillen]

valueToString at lib/FeatureFlag.php:593 formats finite floats with json_encode, whose scientific-notation cutovers differ from serde_json's (what the service compares via to_string_representation): floats in [1e16, 1e17) stringify as plain decimals here but exponentially there (1.0e16 -> 10000000000000000.0 vs 1e+16) and floats in [1e-5, 1e-4) stringify exponentially here but as plain decimals there (0.00001 -> 1e-5 vs 0.00001, 9.9e-5 -> 9.9e-5 vs 0.000099), so exact with filter "1e+16" vs property 1.0e16 returns false (service true) and filter "0.00001" vs 0.00001 returns false (service true).

[arnohillen]

valueToString at lib/FeatureFlag.php:596 falls through to strval for array property values, so property [1, 2] stringifies to "Array" (plus an Array to string conversion warning) instead of the service's "[1,2]", making exact with filter "[1,2]" return false where the service returns true, and filter "Array" return true where the service returns false.

[arnohillen]

unicodeLowercase at lib/FeatureFlag.php:580 uses MB_CASE_LOWER_SIMPLE, which skips the Final_Sigma mapping that Rust str::to_lowercase applies in the flags service (rust/feature-flags/src/properties/property_matching.rs:230), so an exact filter "οδος" against property "ΟΔΟΣ" evaluates false locally (reproduced on php:8.2-cli and php:8.4-cli at be566e7, including through matchFeatureFlagProperties) while the service returns true, and is_not inverts the same way; note native MB_CASE_LOWER does apply Final_Sigma on PHP 8.3/8.4 and only lacks it on 8.2, and the new test pins both directions of the gap via the ["ΟΣ", "ος"] assertFalse and the trailing assertTrue for "ΟΣ" vs "οσ" in test/FeatureFlagLocalEvaluationTest.php.

[arnohillen]

computeExactMatch at lib/FeatureFlag.php:558 has no boolean-coercion step, so exact diverges from the flags service whenever the filter value is boolean-ish: filter false or ["false"] against property "banana", 0 or "" returns false here but true in the service (rust/feature-flags/src/properties/property_matching.rs L212-220), and filter ["true","false"] against property "true" returns true here but false there — pre-existing, but this PR is the exact-match alignment change.