We maintain a variety of libraries on Github that are used in Wikibase, and we would like to start testing our code with PHP 8, as support for this version is on the roadmap for wikidata.org.
Most of these repositories currently use Travis for CI. However, making the change in Travis is not as straightforward (See comment T271109#6739532). In addition, and in accordance with the general direction of ADR 16, we would prefer to add these new tests as GitHub actions.
**Scope:**
[x] [WikibaseDataModel](https://github.com/wmde/WikibaseDataModel) [#836](https://github.com/wmde/WikibaseDataModel/pull/836)
[] [WikibaseDataModelSerialization](https://github.com/wmde/WikibaseDataModelSerialization) [#260](https://github.com/wmde/WikibaseDataModelSerialization/pull/260)
[] [WikibaseDataModelServices](https://github.com/wmde/WikibaseDataModelServices) [#244](https://github.com/wmde/WikibaseDataModelServices/pull/244)
[] [WikibaseInternalSerialization](https://github.com/wmde/WikibaseInternalSerialization) [#135](https://github.com/wmde/WikibaseInternalSerialization/pull/135)
[] [Time](https://github.com/wmde/Time) [#155](https://github.com/wmde/Time/pull/155)
[] [Number](https://github.com/wmde/Number) [#131](https://github.com/wmde/Number/pull/131)
[] [Serialization](https://github.com/wmde/Serialization) [#30](https://github.com/wmde/Serialization/pull/30)
[] [php-vuejs-templating](https://github.com/wmde/php-vuejs-templating) [#21](https://github.com/wmde/php-vuejs-templating/pull/21)
**Acceptance criteria:**
[] All WMDE maintained Wikibase related GitHub repositories run PHP tests in PHP 8 in CI