PHP Extension - Segmentation fault when serialize() is called on a protobuf message

The PHP C extension segfaults when serialize() is called on a protobuf message.

[!IMPORTANT]
This is a unusual use case but the extension should not crash if a proto message is serialize by mistake.

What version of protobuf and what language are you using?
Version: main
Language: php
OS: Linux

What did you do?

<?php
# serialize_segfault.php

require_once (dirname(__DIR__) . '/vendor/autoload.php');

use Foo\TestMessage;

$msg = new TestMessage(['optional_string' => "Hello, World!"]);

serialize($msg);

# Compile the extension 
./php/tests/compile_extension.sh

# Run script
php -d display_errors=on -dextension=../ext/google/protobuf/modules/protobuf.so  serialize_segfault.php

What did you expect to see

# no errors.

What did you see instead?

Segmentation fault

Patch here

Reproduce the crash with serialize_segfault.php using php/tests/compile_extension.sh and the provided PHP extension command. Inspect the PHP C extension path exercised by serialize($msg), then compare the behavior with the referenced patch. Done means the script completes without a segmentation fault or other errors.