Changes
### Breaking changes
* Global tags are now assigned early. This means that users have to make sure to not overwrite `meta`, but only add to it instead, if they don't want to lose global tags. Wrong: `$span->meta = [ 'some' => 'tag' ]`. Correct `$span->meta['some'] = 'tag'`.
* Support for legacy param `DD_TRACE_RESOURCE_URI_MAPPING` has been dropped with the migration to C code, after more than a year of deprecation. Use `DD_TRACE_RESOURCE_URI_FRAGMENT_REGEX`, `DD_TRACE_RESOURCE_URI_MAPPING_INCOMING`, and `DD_TRACE_RESOURCE_URI_MAPPING_OUTGOING ` instead. See [resource mapping](https://docs.datadoghq.com/tracing/setup_overview/setup/php/?tab=containers#map-resource-names-to-normalized-uri) for more details.
### Added
- (PHP 8) Migrate _dd.origin and distributed header sending to internal (#1267)
- (PHP 5) Add headers ZAI implementation (#1308)
- Add support for PHP 8.1 (#1297)
### Changed
- (PHP 7, 8) Internal (root) span initialization (#1329)
### Fixed
- (PHP 5, 7, 8) Remove all lines containing zai_sapi from config.m4 (#1333)
- (PHP 7, 8) Fix unnamed service spans caused by improper handling of `DD_TRACE_ENABLED` (#1332)