New changes in 3.15.0:
* unregister INI entries and fix invalid read on shutdown (#8042)
* Fix PhpDoc comments for message accessors to include "|null". (#8136)
* fix: convert native PHP floats to single precision (#8187)
* Fixed PHP to support field numbers >=2**28. (#8235)
* feat: add support for deprecated fields to PHP compiler (#8223)
* Protect against stack overflow if the user derives from Message. (#8248)
* Fixed clone for Message, RepeatedField, and MapField. (#8245)
* Updated upb to allow nonzero offset minutes in JSON timestamps. (#8258)