[php] Follow PHP8.4 update: declared nullable parameters explicitly

[77web]

close #20241

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@jebentier @dkarlovi @mandrean @jfastnacht @ybelenko @renepardon

[wing328]

@77web thanks for the PR

I ran the petstore integration tests but got errors:

/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php:1389
/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/tests/ResponseTypesTest.php:115
.

15) OpenAPI\Client\ServerVariablesInOperationTest::testInvalidVariableValueInOperation
Failed asserting that exception of type "ParseError" matches expected exception "InvalidArgumentException". Message was: "syntax error, unexpected variable "$httpHeaders"" at
/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php:411
/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php:407
/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php:218
/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/lib/Api/PetApi.php:183
/mnt/c/Users/wing3/Code/openapi-generator/samples/client/petstore/php/OpenAPIClient-php/tests/ServerVariablesInOperationTest.php:56
.

ERRORS!
Tests: 222, Assertions: 105, Errors: 139, Failures: 15, Skipped: 15.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.709 s
[INFO] Finished at: 2024-12-05T13:10:39+08:00
[INFO] ------------------------------------------------------------------------

can you please take a look when you've time?

To run the tests, execute mvn integration-test in the folder samples/client/petstore/php/OpenAPIClient-php

you will need to run a local petstore server: https://github.com/OpenAPITools/openapi-generator/wiki/Integration-Tests

[77web]

@wing328 Thanks, I've fixed that.

[wing328]

good. now all tests passed

PHPUnit 9.6.19 by Sebastian Bergmann and contributors.

...............................................................  63 / 222 ( 28%)
............................................................... 126 / 222 ( 56%)
............................................................... 189 / 222 ( 85%)
.................................                               222 / 222 (100%)

Time: 00:01.325, Memory: 12.00 MB

OK (222 tests, 384 assertions)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  7.011 s
[INFO] Finished at: 2024-12-06T14:28:11+08:00
[INFO] ------------------------------------------------------------------------