Fixed PHP memory leaks and arginfo errors (#8614)

* Fixed a bunch of incorrect arginfo and a few incorrect error messages.

* Passes mem check test with no leaks!

* WIP.

* Fix build warning that was causing Bazel build to fail.

* Added compatibility code for PHP <8.0.

* Added test_valgrind target and made tests Valgrind-clean.

* Updated Valgrind test to fail if memory leaks are detected.

* Removed intermediate shell script so commands are easier to cut, paste, and modify.

* Passing all Valgrind tests!

* Hoist addref into ObjCache_Get().

* Removed special case of map descriptors by keying object map on upb_msgdef.

* Removed all remaining RETURN_ZVAL() macros.

* Removed all explicit reference add/del operations.

* Added REFCOUNTING.md to Makefile.am.

php/composer.json

diff --git a/php/composer.json b/php/composer.json
index 5ea49ed..f712f0e 100644
--- a/php/composer.json
+++ b/php/composer.json
@@ -24,6 +24,7 @@
   },
   "scripts": {
     "test_c": "./generate_test_protos.sh && ./tests/compile_extension.sh && php -dextension=ext/google/protobuf/modules/protobuf.so vendor/bin/phpunit --bootstrap tests/force_c_ext.php tests",
+    "test_valgrind": "./generate_test_protos.sh && ./tests/compile_extension.sh && ZEND_DONT_UNLOAD_MODULES=1 USE_ZEND_ALLOC=0 valgrind --leak-check=full --error-exitcode=1 php -dextension=ext/google/protobuf/modules/protobuf.so vendor/bin/phpunit --bootstrap tests/force_c_ext.php tests",
     "test": "./generate_test_protos.sh && vendor/bin/phpunit tests",
     "aggregate_metadata_test": "./generate_test_protos.sh --aggregate_metadata && vendor/bin/phpunit tests"
   }