fix PHP installation with homebrew on MacOS (#27923)

tools/internal_ci/helper_scripts/prepare_build_macos_rc

@@ -168,18 +168,24 @@ fi
 
 if [ "${PREPARE_BUILD_INSTALL_DEPS_PHP}" == "true" ]
 then
-  # It's required to update the brew because it won't work with the default version Kokoro has.
-  # This can be fragile, though because the future version of brew can break. In that case,
-  # please consider to fix the certain version like https://github.com/grpc/grpc/pull/24837.
+  # It's required to update homebrew because it won't work with the default version Kokoro has.
+  # This can be fragile though because the future version of brew can break - consider pinning
+  # the formulas if that happens (see https://github.com/grpc/grpc/pull/24837).
   brew update || true
+  # macos mojave support for php7.3 brew formula removed MacOS Mojave support here: https://github.com/Homebrew/homebrew-core/commit/d8496609d310a233bfb46272ec7948845f32d7c5#diff-df64282b6e4230b6f35468dfbbd500fcd62b0f93b01b2b161e92f512b7feb1a3
+  # so we need earlier version of the formulata than that. See https://github.com/grpc/grpc/pull/24837 for an example of pinning homebrew formulas to a specific commit.
+  (cd /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core; git checkout 10b1944581fbbdc216057fd70e89dbbe3d04161e)
   brew config
 
-  # Install PHP 7.3 explictly to address missing php header files and
-  # to work well with the pre-installed phpunit 8.4
   brew install php@7.3 || true
-  export LDFLAGS="-L/usr/local/opt/php@7.3/lib $(LDFLAGS)"
-  export CPPFLAGS="-I/usr/local/opt/php@7.3/include $(CPPFLAGS)"
-  export PATH="/usr/local/opt/php@7.3/bin:/usr/local/opt/php@7.3/sbin:$PATH"
+  export LDFLAGS="-L/usr/local/opt/php@7.3/lib ${LDFLAGS}"
+  export CPPFLAGS="-I/usr/local/opt/php@7.3/include ${CPPFLAGS}"
+  export PATH="/usr/local/opt/php@7.3/bin:/usr/local/opt/php@7.3/sbin:${PATH}"
+
+  # the exit code from "brew install php@7.3" is useless
+  # so instead we check if PHP was indeed installed successfully.
+  # Failing early is better than cryptic errors later in the build process.
+  /usr/local/opt/php@7.3/bin/php --version
 
   # Workaround for https://github.com/Homebrew/homebrew-core/issues/41081
   mkdir -p /usr/local/lib/php/pecl